Multiple master fonts are an extension to Adobe Systems' "Type 1" PostScript fonts. Multiple master fonts allowed the ability to select a font style from a continuous range of "axes." Most fonts only support one or two of these variables, which are:
Weight allows the character weight to be modified, typically from light, through regular, to extra bold.
Width allows the character width to be extended or comprised. Although any font can be compressed or expanded by software, the results from a multiple master font are superior. When a font is artificially expanded, the all features are expanded, including the line weight. This means that vertical strokes will be proportionally thicker than the horizontal strokes, giving an uneven appearance. Multiple master fonts with a width axis are designed to scale appropriately.
Optical sizing in Adobe JensonOptical size allows the character shape to be modified based on how large it will appear to the reader. At small sizes, small details such as serifs and thin lines such as stems and typically bolder. The "x-height" (the height of a lower case "x") is also a larger proportion of the total font height, and the characters may be extended slightly. These changes are designed to make small type easier to read. At larger sizes, these details can be finer and the lines more delicate. Note that the optical size is independent from the actual size of the type. It is up to the user to pick the appropriate optical size for the application and viewing environment (for example, a billboard would want to use small optical size even for extremely large text.
Style, the least used of the multiple master axis, allows any other font property to be continuously modified. One such example is changing the serif style from wedge (triangular shaped) to slab (rectangular shaped).
For example, the Myriad multiple master font had two axes: "weight" and "width." This font would include four separate "master designs" of each character: light compressed, light extended, bold compressed, and bold extended. Any weight or width font in between these endpoints can be produced by interpolating between the character outlines of these master designs. The addition of italics requires another four master designs.
Another example is Adobe Jenson, which supports "weight" and "optical size" axes. This font uses three masters to represent the optical size axis, designed for 6, 12, and 72 point type, respectively. This allows the common size of 12 points to be optimized, but requires 6 master designs for roman, and another 6 for italic.
Multiple master fonts have been superseded by OpenType, which provides more support for difference languages and glyphs, but does not offer the continuous controls for character shape. It is increasingly difficult to use multiple master fonts.
Sources: Myriad Specimen Book, Adobe Systems Incorporated, 1992; and Adobe Jenson Specimen Book, Adobe Systems Incorporated, 1996.
