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What is meant by "gutter" in publishing?
The gutter is the inside margin of a printed book. It is called the gutter because books do not lay flat when they are opened: the pages have to bend out from the spine. This creates a "gutter" where the two pages join.
As the pages disappear towards the binding the edge of the page is obscured, so the margin has to be wider on this edge to allow for this. The more pages in a book, the more pronounced this effect, and so the wider the gutter has to be.