trapping

In more detail

A press lays one ink at a time, and the paper travels between them. It comes back to the same place within a fraction of a millimetre, not exactly. Where a red shape sits against a blue background and the two stop at precisely the same line, that fraction shows up as a thin white thread of bare paper along the edge, and the eye finds it at once.

So the trade makes the two colours overlap very slightly on purpose. The join is then covered whichever way the sheet moves. It is measured in hundredths of a millimetre, and where it is done well nobody ever notices it — which is the point of doing it.

What it does on a real document

How much overlap a job wants depends on the press, the paper and the inks, so the printing works is the one who works it out. What the file has to say is simply whether it has been done already: yes or no, written into the document, never left blank.

You are not expected to know the answer. Ask your printing works and copy theirs across; when they do it themselves, which is usual, the answer is no and that is the right answer.

Where it comes from, and why

The word comes from the workshop: the second ink was said to trap the first where they met. It was done by hand on the film that made the plates, edge by edge, and it was one of the slowest jobs in a print works before software took it over.

See it on a finished document

A sheet prepared for the printing works The rules behind this word, and the texts they are read from