A martingale collar is also known as a greyhound collar. The collar was designed specifically for dogs whose necks are larger and thicker than their heads (for example: greyhounds, whippets, italian greyhounds, and other sighthounds). Martingale collars have become popular with other dog breed owners who prefer the gentle control of a martingale collar over a choke chain, as well as 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. The collars are designed to lay flat on the dog’s neck and are often much wider than traditional collars.
This diagram demonstrates the construction of a basic martingale collar and how it works. The pink loop is referred to as the “martingale loop”:

A martingale collar is fitted to the exact size of your dog’s neck when the collar is in the closed position. When properly fitted, the collar will be comfortably loose when it is not in use. When the martingale loop is pulled, the collar should be very snug, but should not constrict the airway or prevent the dog from being able to breathe normally. If properly fitted, a martingale collar is safe to wear everyday. If it is used for training as an alternative to a choke chain, the martingale collar should be adjusted to fit slightly tighter for the time of supervised training only.
There are many great martingale collars at Amazon.com at a variety of price points, sizes, and styles. Why not take a look?