Adobe illustrator 10 Manuale Utente

You can use a separate, dedicated trapping program to create traps automatically, or you 
can use Illustrator to create traps manually.

Artwork without trap and with trap

Appropriate and inappropriate artwork for Trap command 

When overlapping painted objects share a common color, trapping may be unnecessary if 
the color that is common to both objects creates an automatic trap. For example, if two 
overlapping objects contain cyan as part of their CMYK values, any gap between them is 
covered by the cyan content of the object underneath.

There are two types of trap: a spread, in which a lighter object overlaps a darker 
background and seems to expand into the background; and a choke, in which a lighter 
background overlaps a darker object that falls within the background and seems to 
squeeze or reduce the object. 

Spread: Object overlaps background. Choke: Background overlaps object.

You can create both spreads and chokes in the Adobe Illustrator program. 

It is generally best to scale your graphic to its final size before adding a trap. Once you 
create a trap for an object, the amount of trapping increases or decreases if you scale the 
object. For example, if you create a graphic that has a 0.5-point trap and scale it to five 
times its original size, the result is a 2.5-point trap for the enlarged graphic.