OpenType fonts are in their simplest form a collection of glyph designs grouped into a single font file. Each glyph design contains the outline geometry of the individual characters as well as common design elements such as ligatures, contextual alternates, glyph substitution, and stylistic alternates. Additionally, OpenType fonts contain a body of character information that includes common character formatting elements such as text-direction, stylistic sets, casing, and so forth. The OpenType \"head\" (Figure 1.1, OpenType Font Headings) contains the extents of the design in terms of glyph measurements. The \"OS/2 Subset\" (Figure 1.2, OpenType Font Headings) specifies the relationship of the font program to the OS/2 character set included in the font.
The OpenType font can be thought of as a container for the design of many fonts. All of the glyphs of a given font are stored in the same file. The set of design elements in the OpenType font is called its ◉Operating System/2 (OS/2) subset and is defined by the OpenType spec. The main purpose of the OS² Subset is to classify and name characters according to their relationship to other characters (e.g., capitals and lowercase characters are treated as capitals, and so on).
The font file contains, among other things, the outline geometry of each character, common design elements such as ligatures and stylistic sets, and the body of character information that includes common character formatting values such as text-direction and stylistic sets. The font file also contains a coverages table that tells the PostScript interpreter which character on the Macintosh or Windows display should be displayed for a particular design element such as kashida or foils. This feature can reduce the complexity and memory requirements of displaying particular types of characters by reducing the number of individual glyphs that have to be loaded into memory. 7211a4ac4a