When a commercially printed document uses more than one ink on the same page, each ink must be printed in register (perfectly aligned) with any other inks that it butts up against. Otherwise there will be a gap where the different inks meet.

It's impossible to ensure exact registration for every object on every sheet of paper running through an offset litho printing press - it is a glorified mangle. Compensate for misregistration by slightly expanding one object so that it overlaps an object of a different colour.

Knock Out

By default, placing one ink over another knocks out, or removes, any inks underneath to prevent unwanted colour mixing. This prevents a yellow star becoming green when printed on a blue background - the blue plate leaves a white star-shaped hole for the yellow to print clean.

Trapping

Trapping requires that inks overprint, or print on top of each other, so that at least a partial overlap is achieved at the edges. This is often a tiny amount - the QuarkXPress default is 0.144 points, with 72.27 points to the inch. This equates to an overlap of just 2 thousandths of an inch or about 0.05mm

Spreading

Expanding a light object into a dark object. Because the darker of two adjacent colours defines the visible edge of the object or text, expanding the lighter colour slightly into the darker colour maintains the visual edge. A yellow star would be printed slightly larger - 'spreading' into the blue background a small amount.

Choking

When the object is darker than the background colour, the background colour is pulled in around the object to 'choke' it. So in the example below the purple text should be printed the correct size, and the lighter background would tighten around it to 'choke' it.

Misregistration with no trap (left) and with trap (right)