in Design
Why most book interiors are set in a serif
The research on serifs and legibility is less decisive than people think. The reasons book designers still reach for them are more practical.
Open almost any novel on a shop shelf and the text will be set in a typeface with serifs: the small finishing strokes at the ends of letters. Sans serif text is common on screens, in signage and in many illustrated or practical books, but continuous reading in print has stayed largely serif. It is worth being clear about why, because the usual explanation is not the strongest one.
The legibility claim
The explanation most often given is that serifs guide the eye along the line and so make text easier to read. It sounds plausible, and it may be partly true, but the research on the question has never produced a clear result. Studies comparing serif and sans serif text have tended to find small differences or none, and they are hard to run well: typefaces differ in many ways at once, and readers are faster with whatever they are used to.
So it is safer not to lean on legibility. A well-made sans serif, set at a sensible size with good spacing, is perfectly readable over many pages. A poor serif, set badly, is not. The decisions around a typeface matter at least as much as whether it has serifs.
What serif text faces offer in practice
The stronger reasons are practical, and they come from the typefaces themselves. The serif faces used for books are mostly descended from centuries of type made for exactly this job. Their designers were solving the problem of long reading at small sizes, and the best revivals and new designs in that tradition carry the solutions with them.
That shows in the family. A good book face usually comes with a true italic that has its own character rather than a slanted roman, with small capitals, with old-style figures that sit comfortably in running text, and with a range of ligatures and punctuation. Many sans serif families have these too, but among text faces built for books, serif families have historically been more likely to offer the complete set.
There is also texture. Serifs and the modulated strokes that usually accompany them give a line of text a slightly horizontal emphasis and a varied, even colour. On a page of 30 or more lines, that evenness is restful. Many sans serifs have more uniform strokes and rounder, more open shapes, which can look lighter and more spaced out in a dense block of prose.
Convention is a reason too
Readers have expectations. A novel set in a serif looks like a novel before a word has been read, and that familiarity reduces friction. This is not a sentimental point. Anything unexpected on the page draws attention to the typography, and in a novel the typography is meant to stay out of the way.
Convention works in the other direction as well. Cookbooks, design books, field guides and some non-fiction use sans serif text to signal a different kind of reading: scanning, referring, dipping in. A designer choosing a sans serif for a novel is making a statement, and it should be a deliberate one.
When a sans serif makes sense
There are good reasons to break the habit. Short texts, books with many captions or tables, books for young readers learning letter shapes, and some books designed around a strong visual identity can all suit a sans serif. Humanist sans serifs, which keep some of the stroke variation and proportions of handwriting, tend to work better for continuous text than geometric ones.
The question to ask is not which category is correct but which face, at which size and on which measure, will let this particular text be read for hours without anyone noticing the type. For most long prose, the answer is still a serif, for reasons that have more to do with craft and habit than with any single feature of the letters.