Back Up refers to how an image on one side of a printed sheet aligns with the image on the other side. Your print shop can print on one or both sides of a sheet of paper.
If this is going to be a single sheet publication like an advertising insert or a set of instructions, the sides will be centered with even margins all around.
If this sheet is going to be bound in some fashion, say with coil binding or inserted into a three-ring binder, the margin on the left side of the front will be about half an inch wider than the margin on the right.
However, on the back of the page, the reverse is true. The margin on the right will be wider than the one on the left. When the two are backed up, the extra space for holes or binding will be on the same side of the paper.
If you are setting up pages that will be printed back to back, you need to remember that the odd-numbered page will have a larger left margin and the even-numbered page will have a larger right margin.
If you are working with MS Word, you can set this up when you start your document by going to “Page Setup”, then “Multiple Pages”, and select “Mirror Margins”. You just need to designate the outside margin as one width and the inside margin as that width plus half again. With outside margins of 0.5 inches, the inside margin should be 0.75 inches.
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