Martingale collars are required for all our canine customers, because they tighten to give greater control, and reduce the risk of a dog backing out of his collar and escaping. They’re safer and more comfortable than a choke chain that’s always tight. These collars should be used under supervision. they’re not recommended for tie-out. Always remove collars during play, when unsupervised, and before placing them in a crate. Medium for 10-16 inch neck, Large for 14-20 inch neck. 1inch wide.