Adobe Acrobat 7 Professional - Manual (Page 582)

Setting trap appearance
A join is where two trap edges meet at a common endpoint. You can control the shape of
the outside join of two trap segments and the intersection of three traps. The Trap
Appearance section in the New Trap Preset dialog box has two options:
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Join Style controls the shape of the outside join of two trap segments. Choose from Miter,
Round, and Bevel. The default is Miter, which matches earlier trapping results to retain
compatibility with previous versions of the Adobe Trapping Engine.
Trap join examples, left to right: miter join, round join, bevel join
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End Style controls the intersection of three-way traps. Miter (the default) shapes the end
of the trap to keep it away from the intersecting object. Miter also matches earlier trapping
results to retain compatibility with previous versions of the Adobe Trapping Engine.
Overlap affects the shape of the trap generated by the lightest neutral density object that
intersects with two or more darker objects. The end of the lightest trap is wrapped around
the point where the three objects intersect.
Close-up of trap end examples: miter (left) and overlap (right)