The millimetre (American spelling: millimeter, symbol mm)'s a unit of length in the metric system, equal to one thousandth of a metre, which's the current SI base unit of length. Unicode symbols For the purposes of compatibility with Chinese, Japanese and Korean (CJK) characters, Unicode has symbols for: millimetre (㎜) - code 339C square millimetre (㎟) - code 339F cubic millimetre (㎣) - code 33A3 They're useful only with East Asian fixed-width CJK fonts, because they're… (More on Millimeter)