Leading is the distance from one baseline to the next. For a book interior, set it by eye at roughly 1.2 to 1.4 times the type size, check it printed, and adjust until the lines neither collide nor drift apart.
That is deliberately approximate, because the precise rule you have read is not sourced. The familiar "leading should be 120 percent of type size" appears in no study and, contrary to what is widely claimed, not in Bringhurst either.
What Bringhurst Actually Says
The Elements of Typographic Style is the usual authority cited for the 120 percent figure. Its section on vertical spacing says this and no more:
"Vertical space is metered in a different way [to horizontal space]. You must choose not only the overall measure, the depth of the column or page, but also a basic rhythmical unit. This unit is the leading, which is the distance from one baseline to the next."
No number. The multipliers that appear alongside that passage on webtypography.net, which is Richard Rutter's authorised web adaptation, are Rutter's own commentary on web practice, not Bringhurst's rule. They get quoted as Bringhurst constantly.
So the most-cited source for the most-cited number does not contain it.
What the Research Shows
Very little, and what exists concerns low vision rather than normal readers of print. The consistent finding is that the effect is small.
A study of 65 patients with macular degeneration and Stargardt disease found: "The effect of interline spacing on maximal reading speed (MRS) was significant although small; average MRS increased by 7.1 words/min from standard to double interline spacing." Their recommendation was narrow: "Increasing interline spacing is advisable only for very slow readers (<20 words/min) who want to read a few words (spot reading)."
Another, using a simulated scotoma, put the ratio starkly: "Reading speed was only slightly modulated by interline spacing even for the largest range tested: a 26% gain for a 178% increase in spacing."
A third, in AMD patients across spacings from 1x to 4x: "Averaged across subjects, reading speeds for passages were virtually constant for the range of line spacings tested."
The one positive result is foveal and modest: "At the fovea, reading speed increased with vertical word spacing up to about 1.2x to 1.5x the standard spacing." That range happens to bracket normal book practice, which is mildly reassuring but is a small screen study rather than a book design finding.
No peer-reviewed source I could find gives an optimal leading ratio for normally sighted readers of print.
Four Bodies, Four Numbers
If the research is quiet, the accessibility guidance is loud and inconsistent:
| Body | Medium | Leading |
|---|---|---|
| WCAG 2.2 | Screen, reflowable | at least 1.5 |
| APH | 1.25 | |
| ACB | 1.15 | |
| UKAAF | "ideally single line", no number |
WCAG's requirement is explicit: "Line height (line spacing) to at least 1.5 times the font size; Spacing following paragraphs to at least 2 times the font size."
Notice the pattern: the screen standard asks for the most space, and the print bodies ask for markedly less. That split is not arbitrary. Reflowable text at arbitrary widths on backlit screens benefits from more separation than a fixed, well-measured printed page does.
So there is no single "accessibility standard" for leading, and any article quoting one has picked a body without telling you.
UKAAF gives no number but describes the mechanism better than anyone, including the risk in both directions:
"Reducing leading can make text appear to merge between two lines, or at worst cause letters from different lines to touch or cross, making letter recognition difficult. Increasing leading can help with letter shape recognition and improve reading speed. However too much space between lines, words or characters can also make reading harder."
Too much leading is a real failure mode, not just too little. Lines that drift too far apart stop reading as a block of text and start reading as separate lines, and the eye loses the return sweep.
Setting It in Practice
Since no authority gives you a number for print, here is what actually determines it:
Line length. The longer the line, the more leading it needs, because the eye has further to travel on the return sweep. A wide text block with tight leading is where readers lose their place.
x-height. A face with a large x-height fills more of its line and needs more space beneath it. This is the same reason point size is not comparable across typefaces.
Descenders and ascenders. A face with long extenders needs room or the descenders of one line will crowd the ascenders of the next.
Paper. Show-through on thin stock makes tight leading worse, because the lines from the reverse side interleave visually.
The test is mechanical: print a full page, hold it at normal reading distance, and check whether your eye returns cleanly to the start of the next line. If you lose your place, add leading. If the page looks grey and loose, reduce it.
What Deckle Uses
Deckle's default line height is 1.72, set as a multiple of the type size, against a default body size of 11 points.
That is generous by book standards, where 1.2 to 1.4 is more typical, and it is comfortably above WCAG's 1.5 minimum. It errs toward openness, which is the safer error of the two for a screen preview and for readers who need space, at the cost of some pages.
The number is adjustable in the Styler, and the tradeoff is visible: tightening the leading reduces the estimated page count, which narrows the required gutter band and thins the spine. On a long novel, moving from 1.72 to 1.4 is a substantial page saving.
Two related settings interact with it. Justification is on by default, and justified text with tight leading emphasises the vertical rivers of white space that both WCAG and UKAAF warn about. Hyphenation is also on, which reduces those gaps and lets you use tighter leading without the page going patchy.
Common Mistakes
- Citing Bringhurst for the 120 percent rule. He gives no number in that section.
- Quoting one accessibility figure as "the standard". Four bodies give 1.5, 1.25, 1.15 and no number.
- Applying WCAG's 1.5 to a printed page. It is a screen requirement for reflowable text.
- Adding leading without checking line length. They have to be set together.
- Assuming more space is always better. UKAAF warns that too much also makes reading harder.
- Judging it on screen. Show-through and paper colour change the result.
FAQ
Q: What leading should a novel use? A: Roughly 1.2 to 1.4 times the type size, set by eye on paper. No authority gives a print number.
Q: Is 1.5 the accessibility standard? A: It is WCAG's, for screens. The print bodies specify 1.25, 1.15, or nothing at all.
Q: Does Bringhurst recommend 120 percent? A: No. That figure comes from a web adaptation's commentary, not from the book.
Q: Can leading be too loose? A: Yes. UKAAF says too much space "can also make reading harder", and the eye loses the return sweep.
Q: Does line spacing affect my page count? A: Directly and substantially, and through that, the gutter, spine width and printing cost.