Improved Critical [General]
Choose one type of weapon.
Prerequisite: Proficient with weapon, base attack bonus +8.
Benefit: When using the weapon you selected, your threat range is doubled.
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.
A fighter may select Improved Critical as one of his fighter bonus feats.
Source: SRD 3.5 Feats