Every symbol on an engineering drawing is an instruction, and a lot of them look almost exactly like something that means something else. ⌀25 and R25 differ by a factor of two. Circular runout and total runout differ by one arrowhead. A dimension in brackets does not control the part at all, though it sits on the page looking exactly like one that does.

Misread one and the drawing still makes perfect sense. That’s the problem. You find out at inspection, or on the bench, when two parts that should fit do not.

This page is a lookup table for those moments: every Geometric Dimensioning and Tolerancing (GD&T) symbol and engineering drawing symbol you are likely to meet on a mechanical drawing, grouped by the job it does, with the confusions that cause real damage called out under each chart. One thing to settle before you use any of it. Check whether your drawing is issued to ISO or to ASME, because most of these symbols are identical across both, a handful are not, and two of them were withdrawn from ASME in 2018 while remaining perfectly valid under ISO. The standard named in your title block decides which chart applies to you.

Contents:

• GD&T Symbol Chart

• How to Read a Feature Control Frame

• Dimensioning and Feature Symbols

• Welding Symbols

• Line Types

• FAQs

GD&T Symbol Chart

Each of these appears in the first compartment of a feature control frame. The chart covers all fourteen characteristics, including the two ASME has withdrawn, because drawings issued to earlier revisions stay in circulation for decades and you will still meet a concentricity callout on a legacy print.

Treat this as a general reference for identifying a symbol. Your drawing and the standard it cites take precedence over any chart, so where the two disagree, work from the drawing and query it with the originator before you cut metal.

Concentricity and symmetry were removed in ASME Y14.5-2018 because both are punishing to inspect: verifying them means establishing the median points of opposed elements across the whole feature, not simply indicating a surface. Position, profile or runout now carry the same intent on ASME drawings. Both remain valid under ISO 1101, so an ISO drawing may legitimately still use them.

Two pairs are easy to confuse on the page. Circular runout and total runout differ only by a single arrow against a double arrow, but circular runout controls one cross-section at a time while total runout constrains the whole surface. Circularity and cylindricity have the same relationship: one slice against the entire cylinder.

Watch the position symbol. It is a circle crossed by full-width horizontal and vertical lines, not a circled plus sign. The wrong glyph appears in a great many published reference tables, so check the symbol itself before you trust a table.

Modifier Symbols

Modifiers sit inside the feature control frame and change the conditions under which the tolerance applies.

MMC is the one to understand properly, because it is the modifier that pays you back. On a hole, MMC is the smallest permitted diameter, and every thousandth you make the hole larger than that is bonus tolerance added to the position zone. Reach for LMC instead when your risk is breaking through a wall rather than failing to assemble.

Ⓢ needs care on older prints. Regardless of feature size is now the default in both ISO and ASME, so no symbol is needed to invoke it, and the circled S has been withdrawn from ASME. You will still find it on legacy drawings.

How to Read a Feature Control Frame

The frame is the container that ties a characteristic, a tolerance and its datums together. Read it in this order and the rest of GD&T follows.

• Start with the characteristic: The first compartment names what is being controlled, whether that is form, orientation, location, profile or runout. Everything after it is qualification.

• Check for a diameter symbol: A ⌀ before the tolerance value means the zone is a cylinder. Without it, the zone is two parallel planes. On a hole pattern that single character is the difference between a round zone and a square one.

• Read the tolerance value and any modifier: The number is the width of the zone. A modifier following it changes the conditions under which that number applies, and Ⓜ or Ⓛ may release bonus tolerance as the feature departs from its stated material condition.

• Take the datums in the order given: Primary, secondary, tertiary, left to right. That order is the inspection setup sequence, so swapping B and C gives a different part measured a different way.

• Find the basic dimensions: A position tolerance is measured from theoretically exact boxed dimensions. If they are absent, the callout is incomplete and needs querying rather than interpreting.

Datum Symbols

Where the datum triangle touches decides what the datum is. On a surface or extension line it makes that surface the datum. On a dimension line, or aligned with a feature of size, it makes the feature's axis or centre plane the datum. The letters I, O and Q are never used.

Datum targets exist for parts whose surfaces are too irregular to serve as a full datum plane, which in practice means castings, forgings and weldments. A dashed leader tells you the target sits on the hidden side.

Dimensioning and Feature Symbols

⌀ against R is the costliest confusion on this list. ⌀25 is twice the size of R25. Use R only for arcs and partial circles, never for a full cylinder.

Counterbore and spotface get mixed up constantly. A counterbore is deep enough to recess the head so it sits flush or below. A spotface is often only a few tenths of a millimetre deep and exists purely so a fastener head or flat washer beds down square on a cast or curved face.

On blind holes, ↧ gives the full cylindrical depth to where the drill diameter is complete, not the tip of the drill point. The physical hole is always slightly deeper.

Tolerance Notation

ISO 2768-m is the most common class in general engineering, and it governs every dimension on the drawing that has no individual tolerance. Read the title block first, because that one notation sets the baseline accuracy for the whole part. ISO 2768 also has a second part covering geometrical tolerances with classes H, K and L, so a title block may read ISO 2768-mK.

Anything in brackets is a reference dimension. It cannot be used as an inspection or rejection criterion, and if it conflicts with the dimensions that do control the feature, the drawing needs correcting.

Welding Symbols

Establish which standard your drawing follows before you read any welding callout. ISO 2553 and AWS A2.4 put the same information in different places, and reading one as the other puts your weld on the wrong side of the joint.

ISO 2553 System A uses two reference lines, one continuous and one dashed. The arrow side weld goes on the continuous line, the other side weld on the dashed line.

AWS A2.4, and ISO 2553 System B, use a single reference line. The arrow side symbol goes below the line and the other side symbol above it, which is backwards from what most people assume on first encounter.

ISO 2553:2019 permits both systems, so the drawing must declare which is in use. If it does not, ask. A fillet weld on the wrong face is a structural defect, not a cosmetic one.

Supplementary Symbols

Fillet welds are dimensioned two ways and the difference is not cosmetic. Leg length is prefixed z, design throat thickness is prefixed a, and confusing them undersizes the weld by roughly 30 per cent. ISO drawings state a or z explicitly. AWS drawings typically give leg length.

Intermittent welding, written as 4 × 50 (100) for four 50mm welds at 100mm spacing, saves weld metal and reduces distortion. Do not specify it on joints exposed to corrosion or fatigue.

Line Types

Line width and style carry as much information as the annotations. ISO 128-2 designates each type by a two-part code.

Anything drawn in 05.1 is context rather than the part being made. On assembly drawings the mating component is shown for reference and is not yours to manufacture.

Never scale across a break line. The geometry has been removed from the view, not from the part, so the stated dimension is the true one.

FAQs:

Q: How many GD&T symbols are there?

A: Twelve or fourteen, depending on the standard. ISO 1101 recognises fourteen geometric characteristics. ASME Y14.5-2018 removed concentricity and symmetry, leaving twelve. Any reference stating fourteen without qualification is describing ISO 1101 or a superseded ASME revision.

Q: Which two GD&T symbols were removed?

A: Concentricity and symmetry, both eliminated in ASME Y14.5-2018. Verifying either one means establishing median points across the whole feature, which made them slow and expensive to inspect. Position, profile and runout now carry the same intent. Both remain valid under ISO 1101.

Q: What does a triangle symbol mean on an engineering drawing?

A: Attached to a boxed capital letter, it is the datum feature symbol, identifying the feature used as the measurement origin. Where it touches decides what the datum is: a surface, or the axis or centre plane of a feature of size. A separate right-angled triangle on a welding callout is the fillet weld symbol.

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

A: A counterbore is deep enough to recess a fastener head so it sits flush with or below the surface. A spotface is much shallower, often a few tenths of a millimetre, and exists only to give the head a flat, true bearing surface on a cast, forged or curved face. A counterbore hides the head. A spotface gives it somewhere flat to sit.

Q: Is the countersink angle always 90 degrees?

A: No. Metric countersunk screws to ISO 10642 and DIN 7991 use 90 degrees, which is the most common metric callout. Many imperial countersunk screws use 82 degrees. The drawing should state the included angle explicitly, and if it does not, verify it against the fastener you intend to fit.

Q: What is the difference between circular runout and total runout?

A: Circular runout is measured one cross-section at a time, so it captures eccentricity and out-of-roundness at that slice but nothing along the length. Total runout is measured continuously as the indicator traverses the full surface, constraining roundness, straightness, taper and coaxiality together. On the drawing the difference is a single arrow against a double arrow.

Q: Why are ISO and AWS welding symbols placed differently?

A: The standards developed separately. ISO 2553 System A uses two reference lines, arrow side on the continuous line and other side on the dashed line. AWS A2.4, mirrored by ISO 2553 System B, uses one reference line with the arrow side below it and the other side above. Reading one as the other can put your weld on the wrong side of a joint.

Q: What does a dimension in brackets mean?

A: It is a reference dimension, given for information only. It is not a manufacturing requirement and it must never be used as an inspection or rejection criterion. If it conflicts with the dimensions that control the feature, the controlling dimensions win.

Q: What does ISO 2768-m mean in a title block?

A: It is the general tolerance class applied to every dimension with no individually stated tolerance. The letter is the class: f fine, m medium, c coarse, v very coarse. ISO 2768-m is the most common in general engineering. A title block may also cite a geometrical class from part two, giving a combination such as ISO 2768-mK.