ANSI Z133.1 section Z18.104.22.168 reads; Wedge-shaped blocks... to prevent unintentional movement of vehicle should be placed in front of or in back of a vehicle's tires or tracks. If necessary, the chocks can be placed both in front and in the back ofthe tire or track. 9.15 lbs. Rubber. 10" long x 8" wide x 6" tall priced each. Meets Z133.1 standard.