Gutter

The space between two columns, or between facing pages. It keeps neighbours from touching so each can be read on its own.

The gutter is the negotiated distance between columns, or between facing pages, sized so neighbours neither touch nor drift apart. Too narrow and the eye leaps the gap mid-line; too wide and the columns stop belonging to the same page.

On this site the gutter is a token, one measure reused wherever columns meet, which is much of why unrelated pages feel related. In print the book gutter has a further physics: the binding eats into it, so the inner margin is drawn wider than it will ever appear.