print design - Why do some printed documents have those target-like lines in their corners?

Why do some printed documents have those target-like lines in their corners?

Can't think of any concrete examples right now so I just made this:

Answer

They are trim marks used as a guides to cut the paper.

Sometimes, they are double. One set indicates the limit of the bleed and the other indicates where the paper should be cut. The bleed marks are only for reference and not necessary.

If you prepare a layout print-ready, it's usually better to add the trim marks; it clearly indicates what size is you file.

BUT they are also used on offset printing to help verify the registration of all the plates and make sure they are well aligned. That's why often you won't only these trim marks but you'll also see a little wheel on two or each sides of your document.

They're printed using the "registration" color that contains 100% cyan, 100% magenta, 100% yellow and 100% black; this way they appear on every plate!

^{Image source: trim marks - extrabolddesign.com, registration mark - all-o-kino.ru}