A gutter margin is an extra margin added to the side margin, top margin, or inside margins of a document that you plan to bind. It is the space outside of the page margin on the left, top, or inside of the document, and its purpose is to provide extra space for binding documents such as physical books, manuals, or similar printed materials. The gutter margin is typically the area where you might three-hole punch your paper in single-sheet layouts. The gutter margin is on the very inside of both pages in a facing pages layout. The gutter margin is added to the binding side of the page when printing.

To set a gutter margin in Microsoft Word, you can follow these steps:

1. Go to Layout > Margins.
2. Select Custom Margins.
3. In the Gutter box, enter a width for the gutter margin.
4. In the Gutter position box, click Left or Top.
5. Select OK when you finish.

Note that the Gutter position box is not available and determined automatically when you use the Mirror margins, 2 pages per sheet, or Book fold option. While Word for the web will honor a gutter margin set in Word for the Desktop, it only supports changing the Top, Bottom, Left, and Right margins.