Open a well-made book flat on the desk and measure its four margins. The inner margin is usually the smallest, then the top, then the outer, then the bottom.

Every one of those relationships has a reason, and none of the reasons is symmetry.

The inner and the outer

The inner margin is smallest because it is not one margin. It is two. The inner margins of a facing pair of pages read as a single space running down the middle of the spread, and if you set each of them as wide as an outer margin the spread splits into two pages that happen to be near each other.

The outer margin is wider for two practical reasons. That is where the thumbs go, and a reader holding a paperback needs somewhere to put them that is not the text. And the outer margin absorbs the trim: a book is cut after it is bound, and the cut has a tolerance. Set the outer margin tight and some copies in the run will be tighter still, with the ends of lines a hair from the edge.1

The bottom margin

The biggest, and the one people fight about.

A text block centred exactly on the page appears to sit low. This is an old optical effect and you can test it on yourself in a minute: draw a rectangle, put a smaller one in its exact centre, and look. The inner rectangle seems to have slipped.

Giving more space below than above corrects the illusion and makes the text block look centred. Many classical page proportions put the bottom margin at twice the top.

What happens when nobody decides

A page with equal margins on all four sides looks like a document, not a book. That is what you get by default from almost every tool, and defaults are not decisions.

The other failure is margins set by a paper-saving exercise: text almost to the trim, an inner margin half lost in the binding, a bottom margin thinner than the top. It reads as cheap because it is cheap, and a reader who knows none of this can tell. They will not say “the margins are wrong”. They will say the book feels crowded, or that it was hard going, or nothing at all, and not finish it.

And on a screen

A web page has no binding and no trim, so two of the four reasons disappear. The other two do not. The text still needs room to be read as a block rather than as a strip, and the optical centre is still above the geometric one.

On this site the column sits slightly left of centre on a wide screen, with the notes and pull quotes allowed into the space on the right, the way a scholar’s margin fills up in pencil.2 The space around the text is doing work, and the amount of it is a decision somebody either made or failed to make.

Footnotes

  1. A printer friend’s rule of thumb: the outer margin should survive a three-millimetre mistake by the guillotine operator without anyone noticing. ↩

  2. Who gets to fill that margin, and with what, is a question the Reading slowly series keeps coming back to. ↩