A gutter margin setting adds extra space to the side margin or top margin of a document that you plan to bind. A gutter margin helps ensure that text isn't obscured by the binding.
1. On the Page Layout tab, in the Page Setup group, click Margins.
2. Click Custom Margins.
3. In the Multiple pages list, click Normal.
4. In the Gutter box, enter a width for the gutter margin.
5. In the Gutter position box, click Left or Top.
The Gutter position box is not available when you use the Mirror margins, 2 pages per sheet, or Book fold option. For those options, the gutter position is determined automatically.