You can attempt to trip your opponent in place of a melee attack. You can only trip an opponent who is no more than one size category larger than you. If you do not have the Improved Trip feat, or a similar ability, initiating a trip provokes an attack of opportunity from the target of your maneuver.
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.