Martingale dog collars were first used by owners of hunting dogs like Whippets and Greyhounds whose necks are larger than their heads. The unique two-loop construction keeps the dog from backing out of their collar. The larger loop slips over the dog’s head; the smaller loop attaches to the leash and constricts when it feels tension. When there is no tension on the small loop, the collar fits comfortably around the dog’s neck.