What ⧯ means
Its role depends on the surrounding mathematical notation.
In mathematical notation, it is one of six error-barred squares, diamonds, and circles. TeX names this form \errbarblacksquare; the command selects a character, while the formula supplies its role.
Is ⧯ an error icon?
Only when the surrounding notation assigns it that meaning.
The bar is part of the glyph, not an instruction that identifies what went wrong. Its outline can vary with the mathematical font.
Don't confuse it with
4Questions 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 29EF, 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 “error-barred black square”.
Linux
Ctrl+Shift+UType 29ef, then Enter or Space.
HTML
⧯CSS
\29EFCodes and encoding1 code points
HTML decimal
⧯
HTML hex
⧯
CSS content
content: "\29EF";
Each escape is closed by the next backslash; a trailing space is needed before a literal hex digit.
URL encoded
%E2%A7%AF
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.