A character can try to trip an opponent, or otherwise knock him or her down, as an unarmed melee attack. A character can only trip an opponent who is one size category larger than the character, the same size, or smaller.
Making a Trip Attack
If the attack succeeds, make a Strength check opposed by the target’s Dexterity check or Strength check (using whichever ability score has the higher modifier). If the character and the target are different sizes, the larger combatant gets a bonus on the Strength check of +4 per difference in size category. The target gets a +4 stability bonus on his or her check if he or she has more than two legs or is otherwise exceptionally stable. If the character wins, he or she trips the target. If the character loses, the target may immediately react and make a Strength check opposed by the character’s Dexterity check or Strength check to try to trip the character.
Being Tripped (Prone)
Tripping with a Weapon
Some weapons, such as the chain and the whip, can be used to make trip attacks. A character doesn’t incur an attack of opportunity when doing so. If the character is tripped during his or her own trip attempt, the character can drop the weapon to avoid being tripped.