Design Nomenclature / Type
Units per em
Also called UPM, unitsPerEm, em square, units per em
This entry has not been translated yet and is shown in English.
The resolution of a font's internal coordinate grid: how many design units divide one em. TrueType outlines conventionally use 2048, a power of two for clean grid-fitting; CFF/PostScript outlines use 1000. Every metric in the font — sidebearings, kerns, x-height — is stored in these units and scales with the type size.
In practice
The number matters when you're reading font tables or writing kerns by hand. A kern of -20 means nothing until you know whether the em is 1000 units or 2048.
Not to be confused with
- PointThe unit of type size, standardised as exactly 1/72 inch by PostScript and every application since. Historically it was never one thing: the Anglo-American point ran near 1/72.27 inch, the Didot point near 0.376 mm, Fournier's smaller again — which is why old specimens don't convert cleanly to anything.
- EmA relative unit equal to the current type size: at 16px, one em is 16px. It is not the width of an M and never was a measurement of any glyph — it is the height of the body the type is set on, which historically was square at the M's cast width. Every other type metric is expressed as a fraction of it.