"Bohemian Garden" martingale collar:

A martingale collar is made with two loops. The larger loop is slipped onto the dog's neck and a lead is then clipped to the smaller loop. When the dog tries to pull, the tension on the lead pulls the small loop taut, which makes the large loop smaller and tighter on the neck, thus preventing escape. Properly fitted, the collar will be comfortably loose whenever the dog is not pulling against the leash.