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Two broken lines on a drawing. The rhythm of the breaks is the entire distinction.
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.
Both are broken thin lines; the dash-dot rhythm against even dashes is the whole distinction.
| Centre line | Hidden line | |
|---|---|---|
| What it shows | An axis, not an edge | A real edge, concealed from this view |
| Pattern | Long dash, dot, long dash | Even short dashes |
| Is there material there? | Not necessarily | Yes |
| Dimensioned from? | Yes — holes are dimensioned from it | No |
| Relationship (from Symbolwise's records) | Commonly confused with Hidden line — Both are broken thin lines; the dash-dot rhythm against even dashes is the whole distinction.; Part of the same system as Hidden line — Members of the line-type vocabulary where the pattern is the meaning. | Commonly confused with Centre line — Both are broken lines; the dash-dot rhythm is the whole distinction.; Part of the same system as Centre line — The line-type vocabulary. |
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.
The contactless indicator is four nested arcs and marks a payment card or terminal. The N-Mark is a wave-like capital N, is a trademark of the NFC Forum, and marks where to tap a device for near-field communication generally. The underlying radio technology overlaps, which is why the two get used interchangeably even though the marks belong to different bodies.