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Marks made for the people producing an object rather than for the person who ends up with it, which is why almost all of them are trimmed away, hidden in a binding or thrown out with a wrapper. Each has a failure attached: a bleed line taken for a trim line leaves a white sliver down a page edge, a fold direction reversed ruins a run rather than a sheet, a section gathered upside down breaks a staircase nobody was looking at, and a proof signed transfers responsibility for every error still on it.
A fine circle crossed by a full-length cross, used to align printing plates.
Short right-angled lines at each corner, showing where a sheet is to be trimmed.
A strip of ink patches along a sheet edge, used to measure printing quality.
A halving series of sheet sizes, from A0 at one square metre downward.
An emblem identifying the printer of a book, placed on its title page or colophon.
A dashed line outside the trim, showing where a sheet is folded.
An arrow or an underlined dimension on a ream wrapper, showing paper grain.
A small black block on a section's spine, stepping down the book when gathered.
A signed and dated box on a proof, transferring responsibility for the content.