:.· Coordinate system

The dots that make up a glyph are defined in a coordinate system, the 'master grid'. The units are called 'font units'. These font units relate to the ????? of a character. In the old days font units where called 'em-square'. The size of the font unit relative to the em-square determines the resolution of the coordinate system. In other words, the coordinate system's resolution is a function of font-units per em-square.

The more font units per em-square the more detailed a glyph can be. The maximum resolution is preferred for lithographs. Font units per em-square should always be to the power of a multiple of two to maximize outline scaling speed. Apple fonts are defined with 2048 font-units per em-square, for example.
The figure shows two different coordinate systems. The coordinates on the left cannot display the details of the letter 'A'. The coordinate system on the right has a four times as much font-units per em-square and thus comes closer to the original.