Crop marks, also known as trim marks, are lines printed in the corners of your publication's sheet or sheets of paper to show the printer where to trim the paper. They are used by commercial printers for creating bleeds where an image or color on the page needs to extend all the way to the edge of the paper. Printers generally can't actually print to the very edge of the paper, so instead they print on a larger sheet of paper and then trim it down to the correct size. Crop marks are used to define where to trim. So, to print crop marks, you must print on a paper size that is larger than the page size you have set for your publication.
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What are crop marks?