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 hit, and if this is also a hit then the final damage is subject to the critical multiplier for that weapon. Note that the second roll does not need to fall within the threat range, it just needs to be enough to hit the opponent.

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 and Impact enhancements on a magic weapon will double its 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.

Common weapons' threat range
18-20/2:
 * Kukri
 * Rapier
 * Scimitar

19-20/3:
 * Khopesh

19-20/2:
 * Bastard Sword
 * Dart
 * Falchion
 * Great Crossbow
 * Greatsword
 * Heavy Crossbow
 * Light Crossbow
 * Longsword
 * Repeating Heavy Crossbow
 * Repeating Light Crossbow
 * Short Sword

20/4:
 * Heavy Pick
 * Light Pick

20/3:
 * Battleaxe
 * Dwarven Waraxe
 * Greataxe
 * Handaxe
 * Longbow
 * Shortbow
 * Warhammer

20/2:
 * Dagger
 * Dart
 * Club
 * Greatclub
 * Heavy Mace
 * Kama
 * Light Hammer
 * Light Mace
 * Maul
 * Morningstar
 * Quarterstaff
 * Shuriken
 * Sickle
 * Throwing Axe
 * Throwing Dagger
 * Unarmed