Description: Trapping: Trap on a press is the ability of a printed ink to accept the next printed ink compared to how well paper accepts that ink. Registration is when you line all your printed images up over one another. Trapping is a term most commonly used in the prepress industry to describe the compensation for misregistration between printing units on a multicolor press. This misregistration causes unsightly gaps or white-space on the final printed work. Trapping involves creating overlaps (spreads) or under laps (chokes) of objects during the print production process to eliminate misregistration on the press.
History: In QuarkXPress 9 the option was available under Preferences as shown in the picture below:
QuarkXPress 10 either offers you In-RIP Trapping or Composite output, setting trap values in QuarkXPress is no longer needed. The only options available are Default, Overprint & Knockout and these options are available in the colors palette.
In-Rip Trapping: In-RIP trapping moves the trapping to the RIP so that it is done at the last moment. The process is automatic, although it is possible to set up zones to allow different automatic rules for different areas, or to disable trapping for areas previously manually trapped.
Overprint: Overprinting refers to the process of printing one color on top of another in reprographics. This is closely linked to the reprographic technique of 'trapping'. Another use of overprinting is to create a rich black (often regarded as a color that is 'blacker than black') by printing black over another dark color.
Knockout: A knockout is a portion of an image that has been removed. When two colors overlap, they don't normally print on top of each other. The bottom color is knocked out of - not printed - in the area where the other color overlaps.
Knockout type is typically text that is knocked out or reversed out of a dark background so that the type appears in the color of the paper