What ∁ means
∁S means the elements of U that are not in S. U is the full set under discussion; S is a set within it.
Modern set notation took shape across 1880–1920. Authors also mark complements with a line over the set letter or a small superscript c; no single sign is universal.
Formal mathematical notation; use it when U is clear.
Without U, “outside S” is ambiguous.
Questions 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 ∁
Windows (Word)
Alt+XType 2201, then Alt+X to get ∁.
Windows
Win+Rrun charmap for the full table, or use the emoji picker's Ω symbols tab (Win + .).
macOS
Control+Command+SpaceOpen the character viewer and search “complement”.
Linux
Ctrl+Shift+UType 2201, then Enter or Space.
HTML
∁CSS
\2201LaTeX input
In math mode, use `\complement`; some Unicode-aware LaTeX input tools convert the command to ∁.
Codes and encoding1 code points
HTML decimal
∁
HTML hex
∁
CSS content
content: "\2201";
Each escape is closed by the next backslash; a trailing space is needed before a literal hex digit.
URL encoded
%E2%88%81
1
On screen
1
Code points
1
JS .length
3
UTF-8 bytes
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.