The OS/2 usWin* values were initially intended to determine clipping-meaning that any part of the glyph that extends above usWinAscent, or below usWinDescent, could get chopped off. The OS/2 sTypo* values were added later by Microsoft and are the ones most apps should be using today. The hhea values are the historically oldest, originally established by Apple. OpenType has a variety of internal metrics, stored in the hhea and OS/2 tables, that are used by many operating systems and applications to determine default line spacing: However it is important to try and make it as consistent as is reasonable, and to provide good default spacing. There is no technical means of specifying a consistent default line spacing across all environments from within the font. Whenever possible, line spacing should be set explicitly by the user rather than relying on an application’s defaults. Many also have a way to explicitly (manually) set the line spacing to a certain amount. Others, such as InDesign, simply set it at a percentage of the point size, usually 120%. Some programs look inside the font for information. Each program has a built-in way of automatically guessing at a good default line spacing, but those techniques differ. Most word processors and page layout programs have two ways of setting line spacing (how far apart vertically two lines of text will be)-automatic and manual (“exactly”).
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