Wooden ramp named for its shape. Used in many sports such as skateboarding, rollerblading, and bicycle freestyle. Usually a halfpipe is framed in 2x4's and covered in sheets of particle board. A layer of either plywood or paperboard is put over the particle board to smooth out the surface.
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Sorry about the terrible ASCII art, but the I couldn't think of how to describe where the coping goes. The coping is the zeroes in the above "art". It's a metal pipe used to grind or slide across for lip tricks. Most halfpipes also have a channel cut into the middle of one or both sides to make it easier to get started on them.