A seat belt, or safety belt, holds a car driver or passenger firmly in their seat when a collision occurs. This reduces personal injuries by preventing occupants from being thrown about inside the vehicle and crashing into the dashboard or windshield or, in the case of rear passengers, from impact with the rear of the front seats.
The most common type of seatbelt fitted to most vehicles is a three-point harness consisting of a lap belt from one side of the seat to the other, plus a second belt going from the inner lap anchor point up and across the wearer's torso to an anchor point on the inside shell of the vehicle behind and above the wearer's shoulder.
A five-point harness is more common in child safety seats. This has a lap belt, two shoulder belts, and an extra belt going up from the center front of the seat vertically between the wearer's legs. Five-point harnesses are also common in racing cars.
The lap belt is usually on a free-running retracting mechanism, so that any slack is always taken up and the belt fits snugly against the occupant but without restricting general movement during normal driving. When the belt is pulled hard, or if the vehicle is under brakes or cornering, or during a collision, the roller mechanism locks, holding the occupant firmly in the seat during an impact. Some seatbelt mechanisms have a pretensioner built in which actively tightens the seatbelt more firmly when an impact occurs.
It is generally accepted that wearing seatbelts reduces the probability of death or injury in a collision. For that reason, wearing seatbelts at all times when inside an automobile is now mandatory in many countries.