Base Attack Bonus

A base attack bonus is an attack roll bonus derived from character class and level or creature type and Hit Dice (or combination’s thereof). Base attack bonuses increase at different rates for different character classes and creature types.
A second attack is gained when a base attack bonus reaches +6, a third with a base attack bonus of +11 or higher, and a fourth with a base attack bonus of +16 or higher. These extra attacks can typically only be made when taking a Full Attack action.
Base attack bonuses gained from different sources, such as when a character is a multiclass character, stack. As sources gain a fraction of a point of bonus each level, the total base attack bonus of a character with bonuses from different sources needs to be calculated. Directions on how to do so along with a tool for automating the process is provided in the Multiclassing article.