What is gutter margin?

In page layout, the margins around a page are blank borders reserved on each edge so that content isn't crowded or cut off. When a printed document will be bound — stapled, stitched, glued, or spiral-bound — extra space has to be set aside specifically at the edge where the pages join, because the binding process physically consumes part of that edge; this specific extra allowance is called the gutter margin.

In MS Word's Page Setup, the Gutter option adds this extra margin on top of the ordinary margins, and its position follows wherever the binding will be: for a normal single-sided printout it sits along the left edge, while in a facing-pages (mirror margins) layout it automatically shifts to the inside edge of each page, next to the spine. So a gutter margin is defined by the binding location, not by a fixed side of the page — which matches the option that describes it as the margin added to the binding side of the page.

• Adding space only to the left margin (as in one option) is just the single-sided default case, not the general definition — in a facing-pages layout the gutter can sit on the right margin instead.

• Adding space to the right margin does not correspond to where binding normally happens in a standard reading layout, so it would not compensate for anything lost to a binder.

• Adding space to the outer edge of the page describes the margin farthest from the spine, the opposite of where a gutter margin sits.