Weapon Finesse

 
Prerequisite(s): Base Attack Bonus 1+, Proficient with weapon
 
Benefit: With a light weapon, rapier, whip, or spiked chain made for a creature of your size category, you may use your Dexterity modifier instead of your Strength modifier on attack rolls. If you carry a shield, its armor check penalty applies to your attack rolls. A fighter may select Weapon Finesse as one of his fighter bonus feats. Natural weapons are always considered light weapons.
 
Source: SRD