Use knockouts in your design to avoid unpleasant printing surprises

Trapping example
What looks like two overlapping circles prints as a circle and a crescent. | Graphics Dictionary | Alpha Index to Full Dictionary:. # | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | XYZ

In design and printing, using a knockout is the opposite of overprinting. Instead of printing an element in one color on top of another color, the top element is knocked out of the base element so its true color shows. A knockout removes a portion of the bottom image.

When two colors overlap, they don't normally print on top of each other. The bottom color is knocked out—not printed—in the area where the top color overlaps.

If the overlapping colors were printed, you could potentially see the affect the base color on the top element.

Knockout Example

A classic example of this is a yellow circle that partially overlaps a black circle. If the yellow circle overprints the dark circle, the light color is contaminated by the dark ink under it. Instead, the portion of the yellow circle that overlaps the dark circle is used to knock out the dark area underneath in order to maintain consistent color. Even if the black circle overprints the yellow circle, the black with the yellow underneath it appears to be a different color than the black of the rest of the circle unless it is knocked out.

Knockout Relation to Trapping

Knockouts introduce the subject of trapping. When one element is knocked out of another, usually one of the elements is very slightly enlarged in a process called trapping so slight movements of the paper on the press don't reveal a white gap between the two elements.

When a gap appears, the colors are said to be out of registration.

In the example, the yellow circle would be very slightly enlarged to prevent misregistration. The process of trapping knockouts is usually handled by the commercial printing company, although it can be done manually in high-end page payout software.

Contact your commercial printer to see if you are expected to trap your document.

Clarifying Intention

In some cases, an overprint and its accompanying color change may be intended in the design. High-end design software offers opportunities to set levels of transparency to elements with the intent of overprinting other colors. To avoid having a commercial printer's prepress department mistakenly "fix" an intended overprint by creating a knockout, send your digital files to the printer along with a color laser print of the file clearly labeled as to your intent.