Keen weapons have a doubled critical hit threat range, e.g. 20 becomes 19-20, 19-20 becomes 17-20, and 18-20 becomes 15-20. This item trait can be temporarily applied with the Keen Edge (bladed weapons) or Weapon of Impact (blunt weapons) spells. The spell Jagged Tooth can apply this effect to animal companions or Wild Shape forms.

Note: This effect does not stack with the Improved Critical feat. It does stack with weapon master's Ki Critical however.