What is bleed in printing?

Paper moves. It shifts a little as it travels through a press, it expands and contracts with humidity, and a guillotine cutting a stack of several hundred sheets will not land on exactly the same line every time. The tolerance is small — well under a millimetre — but it is not zero, and a white line along the edge of a business card is obvious even when it is a third of a millimetre wide.

Bleed solves it by giving the knife somewhere to land. Any element that is meant to reach the edge of the finished piece — a background colour, a photograph, a coloured band — is extended 3 mm beyond the trim line. If the cut lands slightly outside the intended position it still falls inside printed area, and the result looks correct.

Bleed is not the same as the safety margin, and files often confuse the two. Bleed is outside the trim and gets thrown away. The safety margin is inside the trim and is where text and logos must stay so they are not clipped. A correctly prepared file has both.

Supply bleed as part of the artwork rather than asking the printer to add it: scaling a finished design up by a few per cent to fake bleed changes every dimension in the file, and on a packaging job it will no longer match the dieline.