This martingale collar is made with two loops which are connected with O-ring to attach a lead. When your dog tries to pull his head out of the collar, the tension on the leash pulls the collar taut, which makes escaping almost impossible. When adjusted properly the dog is never choked. The collar stays snug around the dog's neck (just behind the ears) until the pressure is made.
Advantages of this collar:
Take a Look at Metal Collar in 3D