Some dogs such as Greyhounds and Whippets require a Martingale collar because of their thin necks and tapered heads. Regular buckle collars will slip right over their heads. A Martingale collar is a limited slip collar. It has two loops. One large loop which slips over the dog’s head and one smaller loop.  The dog leash is attached to a ring on the smaller loop.  The collar can tighten the amount of the smaller loop. It can partially tighten but it has a limit. This is why it was given the name, limited slip collar.  You can adjust the collar so it slips over your dog’s head. When the collar tightens, the dog cannot slip out of it.  The main advantages of this type of collar is that it fits on the dog’s neck and he can’t slip out of it.  Once the collar is adjusted, you don’t have to loosen or tighten it every time you put in on your dog.