When you look at a book, magazine or brochure, you see text and a border of white space surrounding the text called a margin. There is an additional margin you don't see called the gutter margin or the margin used for binding printed materials.
Gutter margins prevent your text from being hidden by the margin, according to Microsoft. In Word 2010, you can set up a margin gutter for single page for use with a three-ring binder. You can also set up gutter margins for use with bound material like a book. These types of gutter margins are called mirror margins in Word since the left margin mirrors the right.
Single Gutter Location
For a three-ring binder gutter, click on the "Page Layout" tab, then click the "Page Setup" dialogue box launcher in the Page Setup group (it looks like a box with an arrow). Click on the up and down arrows in the "Gutter" field and watch the Preview thumbnail change. When you have the gutter you want, click "OK."
Mirrored Gutter Location
For bound material, click on the "Page Layout" tab, and then click "Margins" in the Page Setup group. Click "Mirrored" from the drop-down menu.