kerning
Kerning is the closing up of the gap between two particular letters that would otherwise look badly spaced.
In more detail
Every letter is drawn with a little room on either side, and for most pairs that room is right. For some it is not: a capital A tucked under the arm of a capital T leaves a hole, and the word looks as though it had a gap in the middle of it.
The cure is to move those two letters closer, and only those two. It is not a general tightening of the line: it is a list of pairs, each with its own correction.
What it does on a real document
The list lives in the typeface file, drawn up by whoever designed it, so nothing has to be measured or guessed.
A document asks for it and the pairs are applied as it is set. The same text set without it is perfectly readable; set with it, it simply looks as a printed page looks.
Where it comes from, and why
The word comes from metal type, where the part of a letter hanging over the edge of its own block was called the kern, and two blocks were filed down so it could sit closer to its neighbour.
Doing it by hand was a compositor's craft; putting the pairs in the typeface file let a machine do the same thing every time.
Glossary
- font
- The drawing of every letter, digit and mark a document writes, held in a file of its own. A PDF carries the fonts it is set in inside itself, which is why it opens looking the same on a machine that has never had them. Leave them out and a reader puts another font in their place, and the layout moves.