How to Read Technical Drawings: A Complete Engineer's Guide

A technical drawing is the closest thing engineering has to a legal contract between designer and manufacturer. It’s more precise than a written specification, easier to interpret than a CAD file and it’s the document that governs manufacture and inspection when anything is in dispute, especially on the shop floor. Understanding how to read one isn’t an optional skill for engineers, it’s a foundational one that many may not have been formally taught.

This guide covers the complete picture: what technical drawings are and why they remain the industry standard, how to navigate the title block, projection methods, line types and how to interpret dimensions and tolerances correctly. There’s also a practical step-by-step reading process and a breakdown of the most common mistakes made when reading technical drawings and how to avoid them.

 

Contents:

• What is a Technical Drawing?

• Why Technical Drawings Are Used (and Still Relevant)

• Engineering Drawing Standards: ISO, BS 8888, ASME and DIN

• How to Read an Engineering Drawing Title Block

• Orthographic Projection: How Technical Drawings Show 3D Objects

• Engineering Drawing Line Types and What They Mean

• How to Read and Interpret Dimensions

• Step-by-Step: How to Read a Technical Drawing 

• Common Mistakes When Reading Technical Drawings And How to Avoid Them

• Key Takeaways

• FAQs

What is a Technical Drawing?

A technical drawing, also known as an engineering drawing or a working drawing, is a formal document that communicates a project’s geometry, dimensions, tolerances, materials, finishes and assembly instructions using standardised symbols and conventions.

It's just as worth noting what a technical drawing isn't. It isn’t an illustration of the component. Though it may contain a visual representation of one, that isn’t the primary purpose of the drawing. A technical drawing is a specification, written to ensure a component matches a standardised set of tolerances and dimensions. It also puts a process in place so that any further components made to the same drawing end up identical.

There are several different types of technical drawings that you might encounter. These include:

• Detail drawings: a single component fully dimensioned.

• Assembly drawings: how parts fit together, with part numbers or a Bill of Materials (BOM).

• General arrangement (GA) drawings: high-level positional overview of a complete product or system.

• Exploded view drawings: spatial assembly order.

• Schematic/wiring diagrams: electrical/hydraulic systems.

For this article, we’ll be working on the first of these, the detail drawing, because it’s the most commonly used and covers all of the fundamentals of reading technical drawings. It’s worth remembering that there are other formats in case you encounter a technical drawing that looks different to what you’re used to. In such cases, consider what the purpose of the drawing is and what it shows, which often makes reading them much easier once you know the basics.

At Accu, technical drawings are provided as standard for all of the precision components in our range. It’s a sensible question to ask why this is necessary when Accu also supplies renders of the products, as well as downloadable CAD files. That’s because there are several reasons why technical drawings are still valuable to engineers.

Why Technical Drawings Are Used (and Still Relevant)

There are several reasons why technical drawings remain, for many, the first document an engineer reaches for.

First is accuracy. A technical drawing is more detailed and precise than a written specification because it shows how the measurements relate to one another and to the features of the part. It can also be taken in quickly, without having to picture the component from a block of text.

Second is portability and ease of use. Technical drawings, unlike CAD files, don’t require any specialised software to view them. They can be attached to an email and viewed like any standard image or printed and readily distributed. This makes them ideal for a variety of applications, such as remote working environments, where internet and computer access are limited, or on the shop floor where it’s not practical to have a computer to hand.

Thirdly, technical drawings are a universal language. The symbols and measurements depicted on them are all standardised, as indicated and communicated by the drawing itself. Because they use ISO, ASME and BS standards (which we’ll go over next), they are understood worldwide.

Finally, technical drawings make designing custom components more straightforward by removing ambiguity. They ensure that the component being designed is perfect before committing to manufacture. Accu’s engineers supply full technical drawings routinely through every stage of our custom manufacture process, allowing engineers to see exactly what their specifications translate to before proceeding to order their components.

Engineering Drawing Standards: ISO, BS 8888, ASME and DIN

The first thing to identify when working out how to read an engineering drawing is to understand which standard is being used for the drawing. The predominantly used standards are:

• ISO (International Organisation for Standardization): The dominant standard globally. The regulations governing this standard include ISO 128 (drawing presentation), ISO 2768 (general tolerances), ISO 1101 (GD&T/GPS) and ISO 286 (limits and fits). ISO 128 has been substantially revised and many of its parts have been renumbered since its inception, but you may still see it cited.

• BS 8888: The UK national standard for technical product documentation. BS 8888 aligns with ISO while adding UK-specific guidance. Still prevalent in UK industry. BS 8888:2020 is the current UK standard and references and incorporates a range of ISO standards.

• ASME Y14.5: The US standard for dimensioning and tolerancing. It’s used widely in aerospace, automotive and American supply chains, so worth considering either the origin or destination of the technical drawing being read. Key differences from ISO include datums and GD&T symbol interpretation.

• DIN standards: German origin, historically significant in European engineering. Some DIN standards have been superseded by ISO equivalents.

Always check the title block first to identify which standard(s) the drawing conforms to. This determines how every symbol and convention on the sheet should be interpreted.

How to Read an Engineering Drawing Title Block

When reading technical drawings, you have to start somewhere. The best and most logical place to begin is with the title block of the drawing.

The title block is the information panel. Think of it like a map key that helps you to orient yourself and understand what you’re looking at. Here’s what the different parts of the title block are and what they mean:

• Part name and/or part number: the identity of the component.

• Drawing number: critical for version control. Always verify you're working from the current, most up-to-date revision.

• Scale: what ratio the drawing is drawn at (e.g. 1:1, 2:1, 1:5).

• Projection symbol: first angle or third angle (covered in depth in the next section).

• Material specification: the base material and grade.

• Surface finish (general): default finish applied to all unmarked surfaces.

• Author, checker, approver and dates: traceability and sign-off chain.

• Company/organisation name and logo

• Units: typically mm in metric drawings. Always confirm.

• Applicable standard: e.g. "Drawn to BS 8888" or "Per ASME Y14.5".

• Tolerances block: general tolerances applied to all dimensions not individually toleranced.

Here’s an example of a real Accu technical drawing. We’ll be referring back to this regularly throughout the article, but check the details above against the title block in the diagram below to see what you can learn from it:

Some of the information in this example technical drawing has been removed for GDPR, but from the title block there’s still a lot we can learn. You can clearly see where details would be provided for the author of the illustration, as well as the date it was drawn. You can see that the fastener depicted is to be made from A2 Stainless Steel and is an M2 x 5 mm Slotted Pan Head Captive Screw. There’s also information on the scale of the illustration, which in this case is labelled as 12:1. If it wasn’t to scale, it would say NTS for Not To Scale.

You can also see, in the title block, the symbol and wording to indicate this particular example is a third angle projection showing the end view and side elevation of the component. We’ll cover what a third angle projection is a little later. You can also find the drawing number and the current revision (here denoted by an A), which helps to make sure you’re working with the most up-to-date version. It’s also worth pointing out the note which says “do not scale drawing”. This instruction tells us not to measure the drawing with a ruler and adjust the measurements according to the scale, as printing can distort some elements. Work with the given measurements only.

One of the key components of the title block is the tolerances block, which you can see in the bottom left corner of the diagram. It shows that all dimensions on the diagram are in millimetres and degrees, as well as the tolerances for those measurements. For example, any measurements that don’t have a decimal place in them have a tolerance of plus or minus 0.25 mm as well as the specific tolerances for angles of various descriptions on the illustration, as well as the roughness of the surface finish. That means a measurement given at 8 mm may range from 7.75 mm to 8.25 mm. This aligns with BS 8888, which governs tolerances on linear and angular dimensions.

Orthographic Projection: How Technical Drawings Show 3D Objects

A technical drawing wouldn’t be worth much without its key component: the drawing. In order to remove ambiguity and present the component diagram in the clearest manner possible, technical drawings employ a system known as orthographic projection.

Orthographic projection is a way of representing a three-dimensional object on a two-dimensional page. Features are projected at right angles with no perspective distortion, so unlike a photograph, every feature is shown at true scale in relation to the rest of the component. It's the system that lets a flat drawing describe a solid part without ambiguity, which is why the views are the heart of any technical drawing.

An object is usually shown in up to three standard views:

• Front (elevation): the view looking straight at the front of the component.

• Plan: the view looking down from above, and sometimes up from below.

• End (end elevation): the view looking at the end of the component, along its axis.

It's not always necessary or practical to show all three. On the example drawing, the screw is shown in just two views: the end elevation (the circular view of the head, where you can see the slot) and the side elevation (the long profile showing the head, shank and thread). A top view would add nothing the others don't already show, so it's left off.

First Angle Projection Vs Third Angle Projection

A quick note on the difference between first angle projection and third angle projection, as both refer to different conventions for how the orthogonal projection is laid out in the diagram. 

The core idea behind them is the same: you’re looking at a 3D object from different directions, with the technical drawing showing them. The distinction lies in how those views are presented on the page.

Let’s work through an example. Here’s an image of a slotted pan head captive screw, much like the one in the technical drawing above. On the drawing, the circular end view sits on the left and the long side-on view sits on the right. Because this is a third angle projection (which is the standard in the United States, Canada, Japan and Australia), you can picture the views as the faces of a box unfolding outwards. To see the head, you look at the screw from the left, and in third angle that view is placed on the left. 

A first-angle projection drawing (the standard across most of Europe) does the opposite. Picture the screw resting on a sheet of glass, with you moving around it to get each view. Whatever face points away from you is projected through the object and lands on the far side. So the view from above ends up below the front view, and the view of the right side ends up on the left. Every view lands on the opposite side from where you looked.

Sounds complicated? Here’s a one-sentence summary to help you remember which is which:

• Third angle: The view goes on the same side as where you stood to look.

• First angle: The view goes on the opposite side from where you stood to look.

If you mix them up, left becomes right and top becomes bottom. A hole that should be on the left side of a part suddenly appears to be on the right. That's the kind of mistake that causes real, expensive problems on the shop floor. It matters in the UK too, because BS 8888 permits both conventions, so you can't assume a British drawing uses one or the other. You have to check.

That’s why the projection symbol in the title block exists. It's the drawing's way of telling you which rule it's playing by. Always check it before you interpret any view. The symbol shows a truncated cone (a cone with its tip cut off) drawn in two views: a set of concentric circles that looks a bit like a target, which is the cone seen end-on, and a tapering profile, which is the same cone seen from the side.

The detail that matters is which way the cone tapers relative to the circles. On a third angle drawing, the narrow end of the cone points towards the circles. On a first angle drawing, it points away. It doesn't matter which side of the circles the cone sits on, that just follows the layout of the drawing. The only thing that tells you the convention is which end of the taper faces the circles, so that's what to check before you read any view.

Engineering Drawing Line Types and What They Mean

The different lines used on a technical drawing are often a source of confusion, representing one of the key areas that causes mistakes when reading one. 

To help clear things up, we’ve included a table below that breaks down what all of the different types of lines on a technical drawing are and their meaning. Some of these appear on the example diagram above, so refer back to that to identify them in situ and see how they are typically used.

How to Read and Interpret Dimensions

Dimensioning conventions are governed by the same standards that all engineering measurements are, but the way they’re shown requires a little explanation. Every length, width, height, hole diameter and angle on a part needs to be communicated unambiguously, but how is that done while still keeping things clear?

When you see a dimension on a drawing, it's made of a few standard parts:

• Extension lines: thin lines that reach out from the edge of the feature being measured. They don't touch the feature itself (there's a small gap) they just mark the start and end of what's being measured. 

• Dimension line: the line that runs between the extension lines, with an arrowhead at each end pointing inward. The measurement number sits on top of or along this line.

• The value: the actual number, in whatever units the drawing uses (almost always millimetres).

Think of it like an indication of where to put the jaws of a caliper around, with tick marks showing exactly what's being measured.

There are a variety of different measurements that you’ll find on a technical drawing, several of which you can see on the example above. Here are the handful of different ones you'll encounter regularly, along with the symbols that are used to denote what’s being measured:

• Linear: straight-line measurements. Length, width, height. The most common kind.

• Diameter (⌀): used for circles and cylinders. The ⌀ symbol tells you the value is the full diameter, not the radius. You'll often see it called out on a view where the feature doesn't appear as a circle at all, such as the side view of a hole.

• Radius (R): used for arcs and rounded corners. R is measured from the centre of the arc to its edge, so it's half the diameter of the full circle that arc belongs to. That's why R10 and ⌀10 describe very different sizes.

• Angular: measures an angle in degrees. Works the same way as a linear dimension, just curved.

It gets a bit more interesting when it comes to how the different dimensions are presented on a drawing, because this is how errors can accumulate. There are three kinds of dimensioning used:

• Chain dimensioning: where dimensions are laid end-to-end, like links in a chain. Easy to read, but each dimension's tolerance adds to the next one. By the time you're five features along, the total variation can be surprisingly large.

• Parallel (baseline) dimensioning: every dimension is measured from the same reference edge, drawn as a stack of parallel dimension lines. Because each feature is located from that one baseline rather than from its neighbour, the tolerances don't accumulate. You can see these on the example drawing above.

• Running (datum) dimensioning: the same principle as parallel dimensioning, measuring everything from a single origin, but drawn more compactly from one marked datum point. It avoids stack-up in the same way.

In practice: if you see a chain of dimensions on a precision component, that's worth querying. It may be an oversight by the designer or it may be intentional, but on a tight part the accumulated tolerance is worth checking before anything gets made.

A Note on Geometric Dimensioning and Tolerancing (GD&T)

Geometric dimensioning and tolerancing (GD&T) is a standardised symbolic language used on engineering drawings to control the form, orientation, location and runout of features on a component. Where conventional dimensions specify size and position using plus/minus tolerances, GD&T goes further by defining exactly how a feature must behave geometrically, not just how big it must be.

In practice, GD&T information appears in a feature control frame: a small box divided into compartments that reads left to right, specifying the geometric characteristic, the tolerance value and the datum references the measurement is taken from. Common characteristics include flatness, perpendicularity, true position and circular runout, each represented by a standardised symbol. For a full breakdown of every GD&T symbol and what it controls, see our GD&T symbols guide.

Step-by-Step: How to Read a Technical Drawing

Now that we have more of an understanding of the various components of a technical drawing and what they mean, we can put all this together and use that knowledge.

As a reminder, here’s the example of the technical drawing from earlier. As you work through the step-by-step guide below, refer back to this drawing so that you can practise using what you’ve learned to read it.

Step-by-step process:

• Start with the title block: identify the part, revision, standard, units, projection method, general tolerances and material.
• Orient yourself with the views: identify the front/plan/end views using the projection symbol and understand how they relate.
• Understand the geometry before the dimensions: trace the outlines of each view. Identify hidden lines and mentally build the 3D form.
• Identify critical features first: find the datums and the tightest-toleranced features. These tell you where the functional heart of the part is.
• Read dimensions systematically: work through each view methodically. Cross-reference across views to confirm your interpretation of every feature.
• Check surface finish and material callouts: these affect process selection, not just final inspection.
• Verify against the BOM (for assembly drawings): confirm all part numbers and quantities are accounted for.
• Flag ambiguities before acting: if anything is unclear, never assume. Query the drawing originator. A five-minute question is worth hours of rework.

Common Mistakes When Reading Technical Drawings And How to Avoid Them

1. Working from the wrong revision

Drawing revisions exist because designs change. They change frequently, especially on components that are still being developed or refined. The risk is that a drawing issued last month may look almost identical to the current version, with the only visible difference being a single updated dimension or a revised tolerance buried somewhere on the sheet.

Before reading anything else on a drawing, check the revision indicator in the title block and verify it against the latest issue you have been formally issued. If you're unsure whether your copy is current, ask. Don't assume. In a live manufacturing environment, keeping a record of which revision was used for each batch of work is both good practice and, in regulated industries, a compliance requirement.

2. Misidentifying the projection method

First angle and third angle projection both use the same views, front, plan and end, but place them on opposite sides of the front view on the page. At a glance, a drawing in first angle and a drawing in third angle can look very similar, particularly if the component is relatively symmetrical, like a screw. The consequence of mixing them up is that left becomes right and top becomes bottom, which can mean a feature that should be on one face of a component ends up being machined on the opposite one.

The projection symbol is in the title block for exactly this reason. Check it every time you pick up a drawing, even if you're familiar with the component. If the symbol is missing or unclear, don't proceed until you've confirmed with the drawing originator which method was used.

3. Treating reference dimensions as inspection requirements

Reference dimensions, shown in parentheses, e.g. (50), are included on drawings for information only. They might show an overall length derived from a series of other dimensions or carry a useful measurement across from a related drawing. They do not define a manufacturing requirement and a part cannot be rejected against them.

The mistake typically happens when someone reads a drawing quickly and applies the same level of attention to bracketed dimensions as to standard ones. If you're conducting inspection or checking a component against a drawing, the rule is simple: if it's in brackets, it doesn't count. If you're creating a drawing and you want a dimension to be inspected, make sure it isn't in brackets.

4. Scaling dimensions from a printed drawing

If a dimension isn't explicitly stated on a drawing, the instinct is sometimes to measure it from the printed sheet and scale it up using the drawing's stated scale ratio. This is always wrong, for two reasons. First, drawings are rarely printed at precisely the stated scale. Printer calibration, paper shrinkage and digital-to-print conversion all introduce small but real distortions. Second, a dimension that isn't stated is a dimension that hasn't been formally controlled. Scaling it produces a number that has no engineering authority behind it.

If a dimension you need isn't on the drawing, raise it with the drawing originator and ask for it to be added. Document the request and the response. Never manufacture or inspect to a dimension that hasn't been explicitly specified.

5. Misreading diameter and radius callouts

The ⌀ symbol denotes diameter, which is the full width of a feature through its centre. The R symbol denotes radius, the distance from the centre to the edge. A ⌀25 bore and an R25 bore are not the same thing: ⌀25 means a 25 mm bore diameter; R25 means a bore 50 mm in diameter. The symbols are visually distinct but when reading quickly, particularly on a dense or complex drawing, it's easy to register the number without properly registering which symbol precedes it.

Any time you read a dimension on a circular or curved feature, make a conscious habit of identifying the symbol before the value. This is especially important on drawings with a mix of holes, counterbores, radii and fillets, where multiple different symbols appear in close proximity.

6. Overlooking tolerance accumulation in chain dimensions

Chain dimensioning, where dimensions are laid end-to-end from feature to feature rather than all originating from a common datum, is straightforward to read but creates a compounding tolerance problem. If each link in the chain carries a tolerance of ±0.1 mm and there are five features in the chain, the worst-case variation between the first and last feature is ±0.5 mm, even if every individual dimension is within its own tolerance.

When reading a drawing that uses chain dimensioning, always work out the worst-case stack-up across the full chain before committing to manufacture or assembly. If the accumulated tolerance is tighter than the assembly can accommodate, it needs to go back to the designer. It's a drawing issue, not a manufacturing one.

7. Ignoring hidden lines

Hidden lines, shown as dashed lines on a drawing, represent features that exist on the component but aren't directly visible in the current view. They might indicate a blind hole, a counterbore, an undercut, a thread or an internal bore running through the part. Because they're not solid lines, they can feel visually secondary and it's easy to absorb them as background rather than reading them as the real features they represent.

Every dashed line on a drawing corresponds to something that has to be there. Before finalising your reading of any view, go back and account for every hidden line. Identify what feature it represents and find it in the other views where it will be visible. If you can't find a hidden line's counterpart in another view, that's a signal to look more carefully, not to move on.

8. Misreading a feature control frame

A GD&T feature control frame contains multiple distinct pieces of information, like the geometric characteristic, the tolerance value, any modifiers and the datum references, all packed into a small box. The most common mistake is reading only the tolerance value and ignoring the rest. The modifier symbols (Ⓜ for MMC, Ⓟ for projected tolerance zone) can significantly change the conditions under which the tolerance applies and the order of datum references determines how the part is constrained for measurement.

Read every compartment of a feature control frame in sequence, from left to right, every time. If an Ⓜ modifier is present, calculate the available bonus tolerance at the actual produced size before drawing any conclusion about conformance. If the datum order seems inconsistent with the function of the part, query it. Datum order errors are a known failure mode on drawings produced by less experienced designers.

9. Not querying a drawing you don't fully understand

This is the most important mistake on the list to avoid and the one that underpins all the others. In a busy workshop, when dealing with a client or under schedule pressure, it’s difficult to admit that something on a drawing isn't clear. The assumption is that querying a drawing looks like a gap in competence. In practice, the opposite is true.

Technical drawings are produced by humans and they contain errors, omissions and ambiguities. Experienced engineers query drawings routinely, it's a normal and expected part of the manufacturing process. Every query raised before work starts is a problem solved cheaply. Every ambiguity that gets resolved by assumption is a potential non-conformance, a scrapped component or a costly rework waiting to happen. If anything on a drawing is unclear, raise it in writing, get a written response and keep both on file.

Key Takeaways

Reading a technical drawing well has less to do with memorising every symbol and more to do with following a consistent order. The engineers who read drawings quickly and without error aren't necessarily the ones who know every convention by heart, they're the ones who check the same things in the same sequence every time. The points below distil that sequence into the habits worth building, from the first look at the title block through to the moment you decide something needs querying.

• A technical drawing is a legal specification, not an illustration. What the drawing says governs manufacture and inspection, regardless of what any CAD file, email or verbal instruction says.

• Always start with the title block. It tells you the standard, units, projection method, material, revision and general tolerances before you've looked at a single dimension.

• Check the revision every time. Working from a superseded drawing is one of the most common and most costly mistakes in manufacturing.

• First angle (European) and third angle (American) projection look similar at a glance but place views on opposite sides of the front view. Confusing them reverses left and right and top and bottom. Always verify the projection symbol before interpreting any view.

• Never ignore hidden lines. Every dashed line represents a real feature, it's simply hidden from the current view.

• Dimensions must always be read from the drawing, never scaled from the printed geometry. If a dimension is missing, query it.

• Chain dimensioning causes tolerances to accumulate across features. On precision components, datum dimensioning is preferable because every feature is measured from the same origin.

• GD&T controls things that dimensions alone cannot. Things like straightness, flatness, perpendicularity, true position and runout. The feature control frame always reads left to right: characteristic, tolerance value, datum references.

• When in doubt about anything on a drawing, query the originator before proceeding. A question takes minutes; a remade component takes days.

Further Reading:

• Mastering Surface Roughness: A Comprehensive Guide

• DIN, ISO and AFNOR Standards: What Are They?

• How to Use Vernier Calipers Like an Engineer

FAQs:

Q: What is GD&T and why is it used on technical drawings? 

A: GD&T (Geometric Dimensioning and Tolerancing) is a standardised system for controlling the form, orientation, location and runout of features on a component, beyond what conventional dimensions can specify.

It is used because coordinate tolerances (±x, ±y) create square tolerance zones that are geometrically inconsistent with round features like holes and shafts. GD&T replaces those square zones with cylindrical or planar zones that reflect the actual functional requirement, allowing more usable tolerance and more accurate communication of design intent. 

In ISO standards the equivalent system is called GPS (Geometrical Product Specifications).

Q: What is the difference between a counterbore and a countersink? 

A: A counterbore (⌴) is a flat-bottomed cylindrical recess machined coaxially with a hole, used to recess the heads of socket cap or button head screws so they sit flush with or below the surface. 

A countersink (⌵) is a conical recess at the mouth of a hole, used to accommodate the tapered underhead of countersunk screws. 

A counterbore has a flat bottom and vertical walls; a countersink has a tapered, conical profile. Both are specified with a diameter and a depth or angle, respectively.

Q: What does ISO 2768 mean on a technical drawing? 

A: ISO 2768 is the international standard for general tolerances. These are the default tolerances that apply to all dimensions on a drawing that do not have an individually stated tolerance. 

It has two parts: ISO 2768-1 covers linear dimensions and angles and ISO 2768-2 covers geometric tolerances. ISO 2768-1 defines four classes: fine (f), medium (m), coarse (c) and very coarse (v). Each specifies a different permissible variation for a given dimension range. 

ISO 2768-m is the most commonly used class in general engineering. When ISO 2768 is used, the applicable class is stated in the title block, though some drawings list explicit tolerance values instead, as the example in this guide does.

Q: What should I do if a dimension is missing from a technical drawing? 

A: A missing dimension must always be queried with the drawing originator. It should never be assumed, estimated or scaled from the printed geometry. Technical drawings are legal specifications and every dimension that governs manufacture or inspection must be explicitly stated. 

Scaling from a printed drawing is unreliable due to printing tolerances and paper distortion. Acting on an assumption exposes the manufacturer to liability for any resulting non-conformance. Document the query and the response in writing before proceeding.

Q: What is the difference between a reference dimension and a standard dimension on a technical drawing? 

A: A reference dimension, shown in parentheses, e.g. (50), is given for information only. It does not define a manufacturing requirement and must not be used as an inspection or rejection criterion. It is typically included to provide useful context, such as an overall length that can be derived from other dimensions or a dimension carried over from a related drawing for ease of reference. 

A standard dimension carries no brackets and is a direct specification requirement. The feature must conform to it within the stated or general tolerance.