Font
Glyph Terms
Font Information
A font defines its EM-Square (or UPM, units per em). An EM-Square is a kind of container where each character will be drawn.
The height of the type piece is known as the ‘em’, and it originates from the width of the uppercase ‘M’ character; it was made so that the proportions of this letter would be square (hence the ‘em square’ denomination).
This square uses relative units. In the browser, relative units are scaled to fit the desired font-size.
In digital type, the em is a digitally-defined amount of space. In an OpenType font, the UPM — or em size is usually set at 1000 units. In TrueType fonts, the UPM is by convention a power of two, generally set to 1024 or 2048.``
Typcial EM-Square sizes are
- For a Postscript Font the em size is set - by strong convention - to be
1000. - For most TrueType Fonts the em size is set to be
2048. Also as a convention, TrueType claims rasterization is faster if the sum is apower of 2.
For a 10 pt type, the 1000 units for instance get scaled to 10 pt. So if your uppercase ‘H’ is 700 units high, it will be 7 pt high on a 10 pt type.
OS/2 measure
OS/2 measure are used on Windows platforms
General measures
General measures are used on Mac platforms
