Oxer

A parallel oxer. Note the highest front and back rails are the same height.

An oxer is a type of horse jump with two rails that may be set even or uneven. The width between the poles may vary.

There are several types of oxers:

Ascending: the front rail is lower than the back rail. This is the easiest for the horse to jump, as it naturally fits into the animal's bascule and incourages a round and powerful jump.
Descending: the back rail is lower than the front rail. This type is not often used, as it can cause an optical illusion for the horse. Many trainers believe it dangerous.
Parallel: both the top front and back rail are even, but the jump is higher than it is wide.
Square: a type of parallel oxer, where the jump's height is the same as its width. This is the hardest type of oxer seen in competition.
Swedish: a "cross-rail" type of oxer, the highest front and back rails of the oxer form an X, so that one section of the jump is lower than the other sections.