Two Weapon style

Dual-wielding with no points invested in Two-Weapon Style gives THAC0 penalties of +4 in your on-hand (right) and +8 in your offhand (left). An explanation of Dual-Wielding: Carrying a second weapon in your offhand automatically gives you an additional Attack per Round, but you only swing your offhand weapon once per round. For instance, if you had an ApR of 3 while holding a Mace, and you picked up a Dagger in your offhand, your ApR would jump to 4, and every round your attacks would be Mace, Mace, Mace, Dagger. If you were Improved Hasted, your ApR would double, and you would now attack Mace, Mace, Mace, Dagger, Mace, Mace, Mace, Dagger, every round. Your overall Speed Factor is determined by the weapon in your on-hand, not your offhand; the number of proficiency points invested in your offhand is actually irrelevant, the game only uses your proficiency with your on-hand weapon when determining combat bonuses. (You should still have at least 1 proficiency point in your offhand weapon, though, even if just for the sake of realism.)