Multiple master fonts
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Multiple master fonts (or MM fonts) are an extension to Adobe Systems' Type 1 PostScript fonts, now mostly superseded by the advent of OpenType. Multiple master fonts contain two or more "masters" — that is, original font styles — and enable a user to interpolate between these masters along a continuous range of "axes." With proper application support, these axes can be adjusted on demand. From one MM font, it is conceivable to create a wide gamut of typeface styles of different widths, weights and proportions, without losing the integrity or readability of the character glyphs.
Aspects of multiple master fonts 
Where available, most MM fonts support one or two (and occasionally three) of the following variables:
- 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 compressed. 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, all the 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 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 are 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 axes, allows any other font property to be continuously modified. One such example is changing the serif style from wedge (triangular) to slab (rectangular).
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.
Application support 
Current application support for these fonts is sparse, if not entirely absent. However, font design tools such as FontLab and FontForge can edit MM fonts, and can export into other font formats as needed. Adobe Type Manager (ATM) is required for MM support on Windows and the "Classic" Mac OS (9 and below).
Free software 
Free-software support for multiple master fonts is offered by the program mminstance, which generates standard PostScript fonts from multiple master fonts. These can then be used in any application that is compatible with standard PostScript type 1 fonts.
Legacy of multiple master fonts 
The multiple master font format has mostly been superseded by OpenType, which provides more support for different languages and glyphs, but does not offer the unique continuous controls for character shape. Typically the OpenType versions of old multiple master fonts include a selection of the most commonly used combinations of axis positions.
Multiple master fonts still serve two purposes:
- As the fallback font format of Adobe Acrobat, multiple master fonts are used as a substitute in place of original fonts in the case of missing fonts. Two such substitution fonts are buried amongst the data resources for Acrobat: Adobe Serif MM and Adobe Sans MM. CourierStd is another fallback font family in Acrobat.
- As a design tool for creating families of fonts; a font designer can create a multiple master font from a base font design and then offer customers a wide number of font variations by building them from the multiple axes of an MM font. E.g. by creating a light version and a heavy version of their font design someone could create a multiple master font with a weight axis and then offer clients any custom weight they wanted. Adobe and others continue to use multiple master technology in font design.
List of multiple master fonts 
All known commercial MM fonts were released by Adobe, unless specified. While these faces are discontinued, all have since been upgraded to OpenType standard or "Pro" formats.
- So MM (Apostrophic Labs)
- Booter MM (Apostrophic Labs)
- Impossible MM (Apostrophic Labs)
- MoveMe MM (Luc(as) de Groot)
- Path 101 (Graham Meade)
- Snott MM (Graham Meade)
- Staid MM (Graham Meade)
- Stub MM (Ray Buetens)
Further reading 
- Adobe Developer Resources – Multiple Master Fonts (archived)
- Adobe – Moving from Multiple Master to OpenType
- Myriad Specimen Book, Adobe Systems Incorporated, 1992.
- Adobe Jenson Specimen Book, Adobe Systems Incorporated, 1996.