In a personal injury case, when more than one party can be accused of failing to act like a normal, reasonable person, that is, when more than one party is negligent, then the issue of comparative negligence, or partial fault, becomes important. For example, suppose an accident involves one car that was speeding in the fog, but the other was driving without its lights on, which is at fault? 

In cases when the negligence (fault) of each party involved contributed in some way to the causing of the accident, the responsibility to the other person(s) is reduced by the others' degree of negligence.  That is, if a jury rules that the driver speeding in the fog was 60% at fault for the accident, while the driver without vehicle lights was 40% at fault, and the driver without lights was supposed to receive $20,000, then his recovery would be reduced by the degree of his fault, in this case, $8,000, leaving his total recovery at $12,000. State laws vary on if the speeding driver would recover anything; in some states the driver who bears more than half the responsibility recovers nothing, not the 40% of his damages.