The inside margins or blank space between two facing pages is the gutter. The gutter space is that extra space allowance used to accommodate the binding in books and magazines. The amount of gutter needed varies depending on the binding method.
In saddled-stitched publications the amount of gutter, as well as the outside margins are adjusted to allow for creep. Gutter is sometimes used to refer to the alley or space between columns of text in a page layout.