The **seat belt** is a crucial device for protecting vehicle occupants in the event of an accident. Its main function is to keep people in their seats and prevent them from being ejected from the vehicle. For it to perform its function effectively, the belt must be **extendable**.

- An **extendable** belt allows it to adjust properly to the user's body, neither too loose nor too tight, which is essential for safety. This ensures that the belt remains in the correct position during an accident, reducing the risk of serious injuries.

- The **chest strap** of the belt should pass over the collarbone and the center of the chest, while the **lap strap** should be over the hip bones. An extendable belt facilitates this precise adjustment, preventing it from slipping or being incorrectly positioned, which could cause injuries.

- Additionally, an extendable belt helps prevent the **submarining effect**, which occurs when the body slides under the belt in an accident. Proper adjustment, made possible by extendability, is key to avoiding this dangerous effect.

For these reasons, a seat belt must be **extendable** to ensure maximum protection and safety for vehicle occupants.