Kerning is not tracking, and neither is spacing
Three controls, three different jobs, and a great deal of type ruined by reaching for the wrong one. Kerning fixes a pair, tracking colours a paragraph, and letter-spacing is what you do to small caps and to nothing else.
Spacing is the sidebearing built into each glyph: how much air the designer left on either side of the letter. It applies always, to every pair, and you do not get to change it without editing the font.
Kerning is a correction to a specific pair. AV would gap without it; To would leave the o stranded. The font carries a table of these, sometimes thousands of them, each one a judgement the designer already made.
Tracking is a uniform adjustment across a run of text. It is the blunt instrument of the three, and it is the one most often misused, because it is the one exposed most prominently in every design tool.
The rule of thumb
Tracking is for display sizes and for capitals. Large text needs slightly negative tracking because the spacing was drawn for reading sizes and looks loose when scaled up. Capitals and small capitals need slightly positive, because their forms are wide and even and they crowd each other.
Body text needs neither. If a paragraph looks too loose or too tight at reading size, the problem is almost always the typeface, the size, or the measure — not the tracking.
Numbers, since the tools never volunteer them
Every design tool exposes tracking as an unlabelled slider, so here is roughly where the slider should stop. In CSS terms, per em of the size being set:
| Setting | Tracking | Why |
|---|---|---|
| Display, 40px up | −0.01 to −0.02 em | spacing was drawn for 17px |
| Headings | −0.005 em | the same effect, in moderation |
| Body | 0 | the designer already decided |
| Small text, captions | +0.01 em | crowded smalls merge |
| Capitals, small caps | +0.05 to +0.1 em | even widths need air |
The asymmetry is the lesson: the corrections above body size are tiny, and the one for capitals is five to ten times larger. Letterspacing lower case remains a firing offence; letterspacing capitals is a duty.
When to override a kern pair
Almost never in text. Sometimes in a headline, where a single bad pair is large enough to be a visible hole, and where you are setting six words rather than six hundred. That is the honest boundary: kerning by hand is a display activity.