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Multiple Master Fonts are an extension to the Adobe font format. providing the ability to interpolate smoothly between several ``design axes'' from a single font. Design axes can include weight, size, and even some whacko notions like serif to sans serif. Adobes' first Multiple Master Font was Myriad -- a two-axis font with WEIGHT (light to black) on one axis, and WIDTH (condensed to expanded) along the other axis. In the case of Myriad, there are four ``polar'' designs at the ``corners'' of the design space. The four designs are light condensed, black condensed, light expanded, and black expanded.
Given polar designs, you can set up a ``weight vector'' which interpolates to any point within the design space to produce a unique font for a specific purpose. So you can get a ``more or less condensed, somewhat black face''.
Multiple Master Fonts can be used on any PostScript printer. Multiple Master Fonts need a new PostScript operator known as makeblendedfont. The current crop of Multiple Master Fonts supply an emulation of this operator so the printer doesn't need this operator.
A short tutorial on Multiple Master Fonts and makeblendedfont appears in PostScript by Example, by Henry McGilton and Mary Campione, published by Addison-Wesley.
Danny Thomas contributes that there are a few PostScript interpreter (version)s which have bugs that appear with the emulation of the makeblendedfont operator used to support Multiple Master fonts. There weren't many exhibiting this problem, though it may have happened even with one Adobe interpreter.