SRD:Improved Two-Weapon Fighting
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Improved Two-Weapon Fighting [General]
In addition to the standard single extra attack you get with an off-hand weapon, you get a second attack with it, albeit at a –5 penalty.
Without this feat, you can only get a single extra attack with an off-hand weapon.
A 6th-level ranger who has chosen the two-weapon combat style is treated as having Improved Two-Weapon Fighting, even if he does not have the prerequisites for it, but only when he is wearing light or no armor.