Letter spacing
Tracking is a size decision. The bigger the type, the closer the letters want to sit; the smaller, the more air they need.
Above about 24px I tighten: on this site display sizes run from minus two to minus four percent of the em, and the largest title sits at minus five. Body text is never tightened. Labels at 11px are left alone too, because I do not set labels in capitals, and positive tracking only exists to space capitals out.
Reading the computed values across the whole site found small text carrying minus 1.8 percent because it inherited a display habit. That is the failure mode: a tracking value that made sense at 48px applied at 13px. The fix was tokens per role, so the tracking travels with the size.
- Display: −2% to −5% of the em, tighter as the size grows
- Body and UI text: normal, always
- Monospace labels in sentence case: normal; there are no capitals to space
When not to: a wordmark or a single word set very large may want a hand-kerned value rather than the rule. That is one place, not a pattern.