Spacing

Tracking is the space between letters. Leading is the space between lines. The words come from metal: tracking meant the thin strips of brass a compositor slid between sorts, and leading meant the strips of actual lead laid between lines to open them up. Both were the same gesture — adding material to buy air — which is why they were never really separate choices.

They are still not separate. Open the tracking and every line gets longer, so the eye has further to travel back to the start of the next one, so the leading has to open too or the return journey gets harder. Close the leading and the lines crowd, so any looseness between letters starts to look like a mistake rather than a texture. Move one slider above and look at what the other one now needs. That is the whole lesson.

There is a threshold, and it is not a matter of taste. When the space between letters grows larger than the space between words, the eye stops grouping letters into words and starts reading them as separate marks. The text is still legible in the sense that you can identify every character. It is no longer readable in the sense that you can take in a phrase at a glance. Push the tracking slider past roughly 0.25em and you can feel the exact point where the sentence stops being a sentence.

Leading fails differently and more quietly. Below about 1.2 the descenders of one line begin to touch the ascenders of the next, and the horizontal channel the eye follows disappears. You do not notice this as collision. You notice it as fatigue, and as losing your place and having to re-find the line you were on. That is the tell: a leading problem shows up as a reading problem several seconds later, which is why it survives so many rounds of review.

For body text, leave the tracking alone. A well-made text face has already been spaced by someone who spent months on it, and at reading sizes your judgement is worse than theirs. The exception is when you change the size: type set large needs its tracking closed slightly, because the spacing that looks right at 16 pixels looks loose at 64. Display sizes usually want a touch of negative tracking, somewhere between −0.01 and −0.03em. Capitals want the opposite, which is the subject of a later lesson.

For leading, start at 1.5 and adjust for measure. A short measure can take less; a long one demands more, because the longer the line the harder the return sweep. Set leading unitless rather than in pixels so it scales with the type instead of fighting it. And set it on the body, not on individual blocks, so the page keeps one rhythm rather than collecting a dozen local decisions that never agreed with each other.