Combat Maneuvers/Trip

Action: in place of a melee attack
Target: one opponent who is no more than one size category larger than you
Feats: Deft Maneuvers, Greater Trip

If your attack exceeds the target’s CMD, the target is knocked prone.

If your attack fails by 10 or more, you are knocked prone instead.

If the target has more than two legs, add +2 to the DC of the combat maneuver attack roll for each additional leg it has. Some creatures—such as oozes, creatures without legs, and flying creatures—cannot be tripped.