Kerning is the adjustment of space between particular pairs of characters so that a word looks evenly spaced rather than awkwardly loose or tight in spots. Some letter combinations leave visually uneven gaps if simply set side by side — a capital “A” next to a capital “V”, or a “T” followed by a lowercase “o”, tend to look as though they are drifting apart. Kerning pulls such pairs closer (or, less often, pushes them apart) so the spacing reads as uniform to the eye.
Kerning is distinct from tracking, also called letter-spacing. Tracking adjusts the space uniformly across a run of text, loosening or tightening every gap by the same amount, and is a global decision about a word or line. Kerning is pair-specific: it depends on which two glyphs are adjacent. A well-made font may contain kerning information for hundreds or thousands of letter combinations.
In digital fonts this information is stored in the font file itself. The OpenType specification defines a “kern” table that, in the words of Microsoft’s documentation, “contains the values that control the inter-character spacing for the glyphs in a font.” The classic format encodes a list of glyph pairs, each with an adjustment value that the rendering engine adds to the default advance width when those two glyphs appear together.
Modern OpenType typography handles kerning more generally through the GPOS table, the glyph positioning table that the specification describes as providing “precise control over glyph placement for sophisticated text layout and rendering.” Pair-adjustment positioning in GPOS subsumes the older kern table and extends it, allowing context-dependent spacing and positioning that also serves complex scripts. Applications and browsers typically apply this kerning automatically, and CSS exposes switches that let web authors turn it on or off.
Kerning is one of the small details that separates ordinary text from polished typography. Readers rarely notice it consciously, but its absence is felt: unkerned headlines look ragged, while properly kerned type reads smoothly. Because the adjustments are baked into the font by its designer, good kerning is part of what one pays for in a professionally made typeface.