Wheel chocks are used for safety and accident prevention.
Chocking, also known as blocking, is done to prevent vehicles from unintentionally moving, like rolling while you are working on the vehicle.
Position the chock snuggly against the tire tread. Wheel chocks must be positioned downhill and below the vehicle's center of gravity.
On a downhill grade, position the chocks in front of the front wheels.
On an uphill grade, position the chocks behind the rear wheels.