Bleed margins are extra space on the outer edges of pages used to ensure that any images that are supposed to go right to the edge of the page actually do in the final printed book.
When a book is printed, it is printed on paper of a slightly larger size than is intended for the final book. Once all of the pages have been stacked and bound together, the top, bottom, and outer edges of the page are trimmed, taking the book down to the intended size – the trim size. This method makes it much easier to ensure that the outer edges of the pages are all properly aligned, and that none of them stick out.
When you prepare a book document at a specific trim size, the extra space needed for this method is added automatically.
However, if you have content in your book – usually images – that are supposed to go right to the edge of the page, you must add this extra space – the bleed margin – yourself in the book document. Then you must ensure that any content that is supposed to go right to the edge of the page overlaps the bleed margin. This means that when your book is printed and trimmed, the content will go right to the edge of the page, and there won’t be any thin white lines where the content didn’t quite reach the edge of the trim size.
For example, if you were making a book where the intended final size is B5 (which is 17.6 cm by 25 cm), and you wanted to have some images or other content in the book that went right up to the edge of the page, you would need to add bleed margins, and your book document would need to be set to larger than B5. Bleed margins need to be added to the top edge, bottom edge, and outer edge (but not the inner edge – as this is the edge that is bound, and so cannot be trimmed). The bleed margin size is usually about 0.125 inches / 3.2 mm, so you would need to set your book document to be 17.92 cm wide and 25.64 cm tall. (The exact bleed margin size depends on the printer, so you must always check.)
Most people who write novels and other fiction do not need to consider bleed, as novels and fiction books often consist of just text, with fairly wide page margins, and don’t often have content that needs to go to the edge of the page. Bleed margins are generally more important in non-fiction.
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