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A circle crossed by a diagonal, in two unrelated fields — and a third look-alike in Scandinavian orthography.
Context decides. On a drawing, before a number, it is a diameter. In mathematics, standing alone, it is the empty set. They are separate Unicode characters (U+2300 and U+2205) and both are routinely typed as the Scandinavian letter Ø or a slashed zero, neither of which is either of them.
Near-identical characters from unrelated fields, substituted for each other constantly.
| Diameter symbol | Empty set sign | |
|---|---|---|
| What it means | This dimension is a diameter | The set with no members |
| Code point | U+2300 | U+2205 |
| Where it sits | Immediately before a number | Alone, as a value |
| Origin of the shape | A circle with a diameter drawn across it | The Norwegian and Danish letter Ø |
| Relationship (from Symbolwise's records) | Commonly confused with Empty set sign — Near-identical characters from unrelated fields, substituted for each other constantly. | Commonly confused with Diameter symbol — Near-identical characters from unrelated fields, substituted for each other constantly. |
Unicode Code Chart: Mathematical Operators (U+2200–U+22FF)
The Unicode Consortium · retrieved 2026-07-30
Authoritative for the character's code point, name and block only. Unicode assigns identity; it does not define what a symbol means in use.
Wikipedia · retrieved 2026-07-31
Tertiary reference. Used for what the notation means in ordinary mathematical practice and for attribution of its introduction, not for a claim that any body defines it.
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.
Look at what follows. A prefix never stands alone — it is glued to a unit symbol, as in kW or kg. A constant stands by itself in an equation. Careful physical writing subscripts the constant as k_B for exactly this reason, and the kelvin's capital K is a third claimant to the same letter.
Length is the tell. A crescendo hairpin is drawn long, below a stave, and spans the passage it applies to — its extent *is* the instruction. A less-than sign is one character wide and sits between two values. If it stretches, it is music; if it fits between two numbers, it is mathematics.