TypoLineGap

TypoLineGap is a font metric stored in the OS/2 table of OpenType and TrueType fonts. It specifies the recommended additional vertical space, or leading, to add between lines of text beyond the font’s ascender and descender. The value is given in font units and is converted to device units according to the font size during layout.

In practice, TypoLineGap, together with TypoAscender and TypoDescender (also in the OS/2 table), defines the typographic leading to be used when calculating line height. It is distinct from the LineGap metric found in the hhea table; different rendering engines and platforms may prioritize TypoLineGap, LineGap, or a combination of both when determining the final line height. As a result, the same font can yield different line spacing across systems if only one metric is considered.

For font designers, TypoLineGap provides a scalable hint for comfortable line spacing without hard-coding a specific pixel value. When exporting fonts, setting a reasonable TypoLineGap helps ensure readable leading across sizes and platforms. Designers typically tailor this metric to align with the font’s visual rhythm and desired appearance, testing at common text sizes to achieve consistent legibility.

See also: OpenType font metrics, OS/2 table, hhea table, Typographic metrics, line height, leading.