When a multicolored design is printed on a press, each color of ink has its own printing plate. These plates are aligned so that they print on top of one another, but sometimes the alignment is inaccurate. This is especially common with cheaper printing, such as that used in newspapers or on fast-food paper products. These inaccuracies can cause small gaps between areas of adjoining colors.

Intended image. The left inset shows a closeup of the text. The plus sign, inset at right, is a registration mark which is printed in every ink color to help line up the printing plates.

Actual printed image. Color plates were not properly aligned, as can be seen by looking at the registration marks. The text barely even looks yellow due to gaps between the two colors.

To help with this problem, a design can be created with traps. A trap is a border around all the shapes on the lighter colored ink plate. If the plates are misaligned this margin for error will still fill in the holes in the darker ink with the lighter one.

Original image with traps. Notice the feint border around the letters. This is extra yellow ink "underneath" the blue ink.

Actual printed image with same inaccuracy as previous example. Notice the text is still mostly yellow despite the error.

Traps are only used at the junction of two colors and only on the lighter colored ink. Black is never trapped because it is the darkest color, while yellow has traps at intersections with every ink.

One side effect noticable in the second example above is how the text seems bolder and "yellower" than the first example. Traps tend to visually enhance color borders.

Many desktop publishing applications have trap creation built-in, and often they'll only be added just before printing so that the original design is not altered.