A wheel chock is a wedge-shaped block made of heavy rubber, plastic, or metal placed snugly against the tires. It is a mandatory safety device whenever a car is jacked up or parked on a slope. Even if the parking brake is engaged, a chock provides a physical fail-safe barrier against gravity. For trucks and heavy vehicles, rubber chocks are preferred because they grip the pavement better than plastic. They usually have a rope handle for easy removal.