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Indices : How-To : Defining UPM (Units Per Em)
Discussions:
Em Square settings? (esp. this and this)
UPM value of 1000 set in stone? (esp. this and this)
OpenType-PS with more than 1000 UPM? (esp. this and this)
Opentype CFF with UPM size 1100 instead of 1000?
UPM value VS Glyph Details
The Em