Triple Chain Semi Choke Martingale Dog Collar 2mm Link Chrome 7 Sizes Made of high quality chrome plated steel for durability and resistance to rust and tarnishing. Limited closure ensures dog's safety. Triple chain, 2.5mm Link. Sizes specified in all the way opened and closed positions. Available in 7 sizes. For proper size please measure the circumference of dog's neck and make sure it is not less than "closed" measurement. Also make sure you choose a size (open position) that will fit over the dogs head.