By default, when you print opaque, overlapping colors, the top color knocks out the area underneath. You can use overprinting to prevent knockout and make the topmost overlapping printing ink appear transparent in relation to the underlying ink. The degree of transparency in printing depends on the ink, paper, and printing method used. Consult your print shop to determine how these variables will affect your final artwork.
Where colors printed from separate plates overlap or adjoin one another, press misregistration can cause gaps between colors on the final output. To compensate for potential gaps between colors in
artwork, print shops use a technique called trapping to create a small area of overlap (called a trap) between two adjoining colors. You can use a separate, dedicated trapping program to create traps automatically, or you can use Illustrator to create traps manually.
You may want to overprint in the following situations: