The Multiple Master font format is an extension of the PostScript Type 1 format. “Multiple Masters” are fonts offering design variations at the extremes of the “design axis”. This design axis represents a given variable property for that font, such as:
The mechanism of Multiple Master fonts is that the two masters at the extremes of the design axis have a fixed design, but you can create any variation between these masters. Typically, Multiple Master fonts have two design axes, which require four masters. These masters can be considered the cornerstones of the matrix.
User-defined variant “in-between”
To create the in-between variants, you require Adobe Type Manager.