A martingale collar is also known as a greyhound collar. The martingale collar was designed for dogs whose necks are larger than their heads (like greyhounds, whippets, italian greyhounds, and other sighthounds); but martingale collars have gained popularity among other breed owners who prefer the gentle control of a martingale collar over a choke chain, owners of dogs who try to back out of their traditional buckle collars, and agility and flyball trainers who want a handle on their dog's collar at all times.
 
A martingale collar is fitted to the exact size of the dog's neck when the dog collar is in the closed position. Properly fitted, the collar will be comfortably loose when not in use. When the martingale loop is pulled, the collar should be very snug, but should not constrict the airway. If properly fitted, a martingale collar is safe to wear everyday, but if used in training as an alterative to a choke chain, the martingale collar should be adjusted to fit slightly tighter for the time of supervised training only. Click the picture for a larger version illustrating how a martingale collar fits on a greyhound.