What ⦱ means
It can mean the complement of the empty set. A complement contains the elements of the chosen universe outside a set; because the empty set has none, its complement is the whole universe for that problem.
The plain empty-set sign ∅ appears in a 1939 book by Bourbaki, the collective name of a group of mathematicians. André Weil said he was responsible for its adoption and traced its form to Ø, a letter used in Danish and Norwegian. ⦱ adds an overbar to that base sign.
Can fonts make ⦱ look different?
Yes; its shape depends on the font. Across fonts, the bar height, spacing, and availability can differ. If the glyph looks unfamiliar, check the formula’s notation before reading it.
Formal, specialist mathematical notation; unusual in ordinary messages.
Do not use ⦱ as an informal sign for “nothing”; it needs a defined mathematical convention.
Don't confuse it with
1Questions people ask
1Why does ⦱ show as a box on some devices?
Because that device's fonts have no glyph for it. The character is still correct and still copies correctly — the box is a missing glyph, not a broken character.
How to type ⦱
⦱\29B1Codes and encoding1 code points
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.