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Two small bracket marks before a hole dimension. Each is a picture of the hole it calls for.
The symbol is a section through the hole. A flat-bottomed bracket calls for a counterbore: a cylindrical widening for a bolt head with a flat underside. A shallow V calls for a countersink: a conical widening for a tapered screw head. Match the shape of the symbol to the shape of the screw head and it is unambiguous.
Both are small bracket-like marks before a hole dimension; square bottom against V bottom.
Drawn from the visual attributes recorded for each symbol. Only traits the two records disagree about are listed.
| Trait | Counterbore symbol | Countersink symbol |
|---|---|---|
| Built from | squares, lines and dots | triangles, lines and dots |
| Counterbore symbol | Countersink symbol | |
|---|---|---|
| Shape of the widening | Cylindrical, flat-bottomed | Conical |
| Suits which head | Flat-bottomed — socket cap, hex | Tapered — countersunk screws |
| The symbol | Squared-off U | Shallow V |
| Extra dimension needed | Depth | Included angle |
| Relationship (from Symbolwise's records) | Commonly confused with Countersink symbol — Both are small bracket-like marks before a hole dimension; square bottom against V bottom. | Commonly confused with Counterbore symbol — Flat bottom against angled bottom. |
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol instructs and which standard governs it. Where a standard is named it is reported by this source rather than read, so no meaning is typed `official` on that basis.
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol instructs and which standard governs it. Where a standard is named it is reported by this source rather than read, so no meaning is typed `official` on that basis.
Alpha is a Greek letter and stands for a quantity — an angle, a coefficient, a significance level. The proportionality sign is an operator and stands between two quantities to say they keep a constant ratio. The drawings differ in that the proportionality sign's right-hand stroke runs out in an open curve rather than closing into the letter's tail.
Look at the left-hand edge. A straight flat back is an AND gate: the output is true only when every input is. A curved concave back is an OR gate: true when any input is. Everything else about the shapes is similar, which is why the back edge is worth checking deliberately rather than at a glance.
Equal-length lines mean a capacitor. Alternating long and short lines mean a battery, with the long line positive. If the second line is curved rather than straight it is a polarised capacitor — still a capacitor, but one that must be fitted the right way round.
Look at the pattern. Even short dashes throughout is a hidden line: an edge that exists but is concealed behind material. Long dashes alternating with dots is a centre line: an axis of symmetry or the centre of a circle, which is not a physical edge at all. Reading a centre line as an edge is the commonest beginner's error on a drawing.