Threat range

Each weapon has a threat range for critical hits. If an un-modified (no bonuses applied) attack roll falls within this range for the weapon being used, then there is a chance to do additional damage. A second roll is then made to confirm the critical, and if this is also a hit then the final damage is subject to the critical multiplier for that weapon.

All weapons create a critical threat on a roll of 20, but some have a larger threat range, extending down to 19-20 or even 18-20. Additionally, the Keen enhancement on a magic weapon will double it's threat range, and the Improved Critical Feat can do the same for all weapons in the matching category (however, those two effects do not stack). A Keen Kukri, for example, has a threat range of 15-20.