Car safety is a must, and this is a perfect solution for those mutts who like to be on the back seat. This simple car seat restraint plugs into your seat belt closer, and will attach to any brand of harness or even to the dog’s collar. The difference in sizes just relates to the strap’s size and length – bigger dogs will need a longer strap. We leave ours in the car at all times, so there is no chance of forgetting it or it getting lost around the house.