What ㎜ means
To fit the unit into one East Asian character cell.
Before Unicode, standards for Chinese, Japanese, Korean, and Taiwanese character sets included ㎜ beside ㎝ and ㎞. The box let its two letters occupy one square cell; the unit stayed the same.
Don't confuse it with
2mm
㎟Plain SI spelling for millimetre, without the box.
Square Mm Squared
㎜ vs ㎟: ㎟ means square millimetre (area); ㎜ means millimetre (length).
Questions people ask
1Why did ㎜ become mm?
Compatibility normalization can turn the boxed character into `mm`, removing its one-cell appearance but not changing the measurement.
How to type ㎜
Windows (Word)
Alt+XType 339C, 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 “square mm”.
Linux
Ctrl+Shift+UType 339c, then Enter or Space.
HTML
㎜CSS
\339CCodes and encoding1 code points
HTML decimal
㎜
HTML hex
㎜
CSS content
content: "\339C";
Each escape is closed by the next backslash; a trailing space is needed before a literal hex digit.
URL encoded
%E3%8E%9C
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.