In 3.5, if you roll a 20, or lower for certain weapons, you have a Critical Threat; you have struck a vital area, and can deal large amounts of damage. Whenever you get a critical threat, you roll again to confirm your success against the same AC. If you succeed, your damage is multiplied based on the weapon, usually x2. In 3.6, you also get a bonus effect based on a d12 for location and d6 for effect, off our crit table. Also in 3.6: if you roll a 20 on both the original roll, and the confirm roll, you get a double crit: the damage is multiplied by the crit mod twice. Aka x2, then x2 again. If you roll 3 20s in a row, the target instantly is killed by the attack. In 3.5 if you roll a natural 1 on any check you cannot add any bonus and it is a catastrophic failure, typically leading to a setback in the effort. In 3.6 you can also use this table to determine what sort of setback you face if you roll a natural 1 in combat.