This Martingale-style design is a favorite with Obedience, Agility, and Flyball trainers. When used with a leash, if you or the dog pulls on the leash, the collar tightens, increasing control and reducing the risk of "escape" from the collar. It also is a great collar for off lead work because its style allows for easy grasp of the top loop. For correct fit, measure around your dog’s neck at the TOP of the neck.