Keen

A Keen weapon is sharper than usual, making it easier to make critical hits.

Additional information

 * 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, but does not have a threat range of 9-20 if it's owner have taken the Improved critical feat.
 * The Impact weapon enhancement doubles the critical range for bludgeoning weapons.