in Design
Leading: the space between the lines
Leading takes its name from strips of metal, but its job has never changed: to let the eye find the next line without effort.
Leading is the vertical distance from one baseline of text to the next. The word comes from metal typesetting, where compositors placed thin strips of lead between lines of type to open them up. The term has outlasted the material, and it is still pronounced to rhyme with bedding.
Designers describe type size and leading together. Text set at 11 on 14, written 11/14, means 11-point type with 14 points from baseline to baseline. Software often expresses leading as a percentage of the type size instead, but the principle is the same.
What leading does
Leading has one job: to let the eye travel back from the end of one line and land cleanly at the start of the next. Too little, and ascenders and descenders crowd each other, lines blur together, and readers occasionally re-read a line or skip one. Too much, and the lines drift apart, the text looks thin and loose, and the page stops reading as a single block.
The right amount depends on several things at once, which is why no single number works for every book.
What the right amount depends on
The typeface matters first. A face with a large x-height has less space between the lowercase letters of adjacent lines at the same leading, so it usually needs more. A face with a small x-height and long ascenders and descenders already carries a lot of internal space and can be set a little tighter.
The measure matters next. Longer lines need more leading, because the eye has further to travel on the return sweep and more chance of losing its place. Short lines can be set closer. If you widen the text block, you will probably need to open the leading too.
Weight and colour matter. Dark, heavy faces benefit from more air; light faces can look washed out if the leading is too generous. So does the reader: books for young readers and large-print editions are usually set with more generous leading as well as larger type.
As a rough starting point, many book designers find that leading of around 120 to 145 per cent of the type size suits continuous prose. That is a range to test from, not a rule.
Baseline grids and facing pages
In a well-made book, lines of text on facing pages align with each other, and ideally they align through the paper too, so that lines on the back of a leaf sit exactly behind the lines on the front. This is called backing up, and it matters more than it sounds, because thin paper lets the reverse show through.
To achieve it, the leading is set as a fixed value and everything else is built on multiples of it. Space above and below headings, space around block quotations and scene breaks, and the depth of the text block itself should all add up to whole lines, or the grid falls apart as soon as the first heading appears.
This is also why leading should be decided early, alongside the typeface and the page size. The number of lines per page follows directly from it, and that in turn affects the page count.
How to judge it
Print a full page at actual size on paper similar to the final stock. Read it, rather than inspecting it. Then try the same page with a quarter of a point more and a quarter of a point less. Small differences are surprisingly visible in print, and the best setting is usually the one where the page feels calm and the lines feel connected without touching.
If you notice the leading at all, it probably needs adjusting.