Illustrator: What is Trapping? How to do trapping? I

About traps
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.

Note: When artwork does contain common ink colors, overprinting does not occur on the shared plate.

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.

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.

Trapping with tints
When trapping two light-colored objects, the trap line may show through the darker of the two colors, resulting in an unsightly dark border. For example, if you trap a light yellow object into a light blue object, a bright green border is visible where the trap is created.
To prevent the trap line from showing through, you can specify a tint of the trapping color (here, the yellow color) to create a more pleasing effect. Check with your print shop to find out what percentage of tint is most appropriate given the type of press, inks, paper stock, and so on being used.

Trapping type
Trapping type can present special problems. Avoid applying mixed process colors or tints of process colors to type at small point sizes, because any misregistration can make the text difficult to read. Likewise, trapping type at small point sizes can result in hard-to-read type. As with tint reduction, check with your print shop before trapping such type. For example, if you are printing black type on a colored background, simply overprinting the type onto the background may be enough. To trap type, you can add the stroke below the fill in the Appearance palette, and set the stroke to overprint (or set it to Multiply blending mode).

Using the Trap command
The Trap command creates traps for simple objects by identifying the lighter artwork— whether it’s the object or the background—and overprinting (trapping) it into the darker artwork.

Note: The Trap command is only available when you are working on CMYK documents.

You can apply the Trap command as a Pathfinder command or as an effect.

In some cases, the top and bottom objects may have similar color densities so that one color is not obviously darker than the other. In this case, the Trap command determines the trap based on slight differences in color; if the trap specified by the Trap dialog box is not satisfactory, you can use the Reverse Trap option to switch the way in which the Trap command traps the two objects.

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(Special Note: all the figures regarding this topic "trapping" are at the end of this guidance trip.)