Improved Critical( Player's Handbook v.3.5, p. 95)
[Fighter Bonus Feat, General]
Choose one type of weapon, such as longsword or greataxe. With that weapon, you know how to hit where it hurts.
base attack bonus +8, Proficient with weapon,
Devastating Critical (ELH) , Eviscerator (LM) , Keen Strike (ELH) , Overwhelming Critical (DMG) , Overwhelming Critical (ELH) , Vorpal Strike (ELH) , Overwhelming Critical (Dr) , Staggering Blow (DC) , Staggering Critical (DrU) , Devastating Critical (Dr) , Twist the Knife (EE) , Power Critical (MW) ,
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).
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 (see page 38).