d20 SRD said:If you wield a second weapon in your off hand, you can get one extra attack per round with that weapon. You suffer a -6 penalty with your regular attack or attacks with your primary hand and a -10 penalty to the attack with your off hand when you fight this way. You can reduce these penalties in two ways:
If your off-hand weapon is light, the penalties are reduced by 2 each. (An unarmed strike is always considered light.)
The Two-Weapon Fighting feat lessens the primary hand penalty by 2, and the off-hand penalty by 6.
Circumstances Primary Hand Off Hand
Normal penalties -6 -10
Off-hand weapon is light -4 -8
Two-Weapon Fighting feat -4 -4
Off-hand weapon is light and
Two-Weapon Fighting feat -2 -2
d20 SRD said:Strength Bonus
When you hit with a melee or thrown weapon, including a sling, add your Strength modifier to the damage result. A Strength penalty, but not a bonus, applies on attacks made with a bow that is not a composite bow.
Off-Hand Weapon
When you deal damage with a weapon in your off hand, you add only ½ your Strength bonus.