in Design
Margins, and why the inner one matters most
Margins frame the text, give thumbs somewhere to go and, at the spine, decide whether a reader can see the words at all.
Margins are the space around the text block. They are easy to treat as leftovers, whatever is not text, but they do several jobs, and in a bound book one of them is structural.
A printed page has four margins: the head at the top, the foot at the bottom, the fore-edge on the outside, and the inner margin next to the spine, also called the gutter or back margin. On a spread, the two inner margins sit side by side.
What margins do
They frame the text. A generous margin gives the page a calm, deliberate look; a thin one makes it feel crowded and cheap, whatever the typeface.
They give the reader somewhere to hold the book. Thumbs rest on the fore-edge and sometimes the foot. If the margins are too narrow, readers cover words with their hands.
They protect the text from the trimming process. Pages are printed on larger sheets and trimmed to size, and trimming is never perfectly exact. Text set too close to the edge risks looking uneven from page to page, or being trimmed.
The classical proportions
Historically, many book designers worked with proportions in which the inner margin is the smallest and the foot the largest, with the head and fore-edge in between. The reasoning is that on an open spread the two inner margins read together as a single band between the two text blocks, so each can be smaller. A larger foot gives the text block a sense of sitting slightly high on the page, which the eye tends to read as balanced rather than sinking.
Some of these systems have well-known geometric constructions. The Van de Graaf canon, as reconstructed in the twentieth century, and Jan Tschichold’s writing on the proportions of medieval manuscripts and early printed books are the usual references. They are worth studying, although few modern trade books follow them exactly, since they use a lot of paper.
Why the inner margin matters most
The classical idea that the inner margin can be smallest assumes a book that opens flat. Most modern paperbacks and many hardbacks do not. In a perfect-bound paperback, the pages are glued at the spine, and the paper curves into the binding as the book opens. Part of the inner margin disappears into that curve.
How much depends on the binding, the paper and above all the thickness of the book. A slim book opens fairly flat. A thick one may hide a considerable width at the spine, and a reader will have to force it open to see the ends and beginnings of lines. Nothing about the typography matters if the words are in the gutter.
So in practice the inner margin is the one to set first and set carefully. Designers often increase it for longer books and reduce the outer margin to compensate, so that the text block moves slightly away from the spine. Printers will often advise on a minimum gutter for a given binding and page count, and it is worth asking.
Checking the result
The only reliable test is physical. Print a spread at actual size, fold it, and if possible look at a bound book of similar format, paper and extent. Hold it as a reader would, one-handed as well as two. Check that the ends of lines on the left page and the starts of lines on the right are fully visible without pressing the spine.
Then look at the whole spread from a little distance. The two text blocks should sit as a pair, with the space between them feeling like part of the design rather than a gap.
Good margins are noticed only when they are missing.