Needs review
Source: Player's Handbook

Description: Choose one type of weapon, such as longsword or greataxe. With that weapon, you know how to hit where it hurts.

Prerequisites: Base Attack Bonus +8, Weapon Proficiency

Required for: Devastating Critical, Overwhelming Critical

When using the weapon you selected, your threat range is doubled. For example, a longsword usually threatens a critical hit on a roll of 19-20 (two numbers). If a character using a longsword has Improved Critical (longsword), the threat range becomes 17-20 (four numbers).

Special

You can gain Improved Critical multiple times. The effects do not stack. Each time you take the feat, it applies to a new type of weapon. This effect doesn't stack with any other effect that expands the threat range of a weapon (such as the keen edge spell). A fighter may select Improved Critical as one of his fighter bonus feats.