What is trapping in printing?

Every colour in an offset job is printed by a separate plate on a separate unit, one after another. Between units, the sheet is gripped, released and re-gripped. The register is extremely good but it is not perfect, and a shift of a fraction of a millimetre is normal across a long run.

Where two solid colours butt against each other, that shift has nowhere to hide. If cyan moves a hair to the left of the magenta beside it, the paper shows through as a white hairline down the join — obvious, and impossible to fix once printed.

Trapping removes the possibility by making the two colours overlap slightly to begin with. The lighter colour is usually spread very slightly under the darker one, because the darker colour dominates the overlap visually and the change is invisible. If the sheet then moves, it moves within the overlap and no paper is exposed.

It is handled in prepress rather than in the design, and you do not need to build it into artwork — attempting to do so by hand usually causes more problems than it solves. What is worth knowing is that designs with fine light type on a dark solid, or intricate colour-on-colour detail, are inherently more demanding, and that showing them on a physical proof is the only reliable way to see how they will hold.