Mirror-image symbols for genuinely incompatible conventions. Reading a drawing under the wrong one inverts it.
Look at which side the circles are on. In the first-angle symbol the end view sits to the left of the cone; in third-angle it sits to the right. That mirroring is the whole message: the two conventions place every view on opposite sides, so a part made from a drawing read under the wrong convention comes out mirrored. Which applies is stated on the drawing precisely because it cannot be guessed.
The symbols are mirror images and are read at a glance, which is exactly when a mirror image is missed.
Drawn from the visual attributes recorded for each symbol. Only traits the two records disagree about are listed.
| Trait | First-angle projection symbol | Third-angle projection symbol |
|---|---|---|
| Orientation | upright | mirrored |
| First-angle projection symbol | Third-angle projection symbol | |
|---|---|---|
| Where each view is placed | On the side away from the observer | On the side nearest the observer |
| Circles in the symbol | To the left of the cone | To the right of the cone |
| Dominant in | Europe and much of Asia | North America |
| Consequence of confusing them | The drawing reads mirrored | The drawing reads mirrored |
| Relationship (from Symbolwise's records) | Commonly confused with Third-angle projection symbol — The symbols are mirror images and are read at a glance, which is exactly when a mirror image is missed.; Opposite of Third-angle projection symbol — The two conventions arrange views on opposite sides; the symbols are mirror images. | Commonly confused with First-angle projection symbol — Mirror images read at a glance.; Opposite of First-angle projection symbol — The alternative arrangement. |
Multiview orthographic projection
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.