What is trapping in printing?

What it means for your file

The lighter color is usually spread into the darker one, because the darker color defines the visible edge. Small black type set to overprint is a simple trap. Most printers apply trapping in their own prepress software, set for their press, so designers rarely add traps unless asked.

Trapping exists because of registration, how precisely the plates line up. Small type in a single ink can’t misregister against itself, so it needs no trap.

An example

A yellow logo on a dark blue background is knocked out of the blue. If the plates shift a fraction of a millimeter, a thin white line opens along one edge. A trap spreads the yellow very slightly under the blue, so the overlap hides the shift. Enlarging a light object into a darker background is a spread; when the object is the darker one, the background is spread into it instead, which is called a choke.

A common mistake

Adding traps by hand to a file the printer will trap anyway, or building strokes to fake them, so the overlap doubles and dark edges appear. Ask first: most printers want untrapped files, and those who don’t will give their trap width. Screen printing and flexography usually need larger traps than offset printing.