A fundamental probability rule is:

If events A and B are *independent*, meaning that one event occurring does not affect whether the other event occurs, then this formula can be simplified to:

An example of two independent events are two consecutive fair coin flips: the outcome of one coin flip does not affect the other coin flip outcome. By contrast, two dependent events would be driving intoxicated and getting into a car crash: the probability of getting into a car crash significantly increases if someone is driving drunk.