Martingale Collars are a great light training tool that adds pressure if tension is applied to the leash, and releases pressure as tension releases. Martingale collars are a one-size fit, but can be made adjustable if a buckle is added. They are very strong and are unlikely to break during play or if sudden tension is added. I love martingale collars and always use them with foster dogs, especially because we do not have a fenced yard and use a tie-out - the last thing we want is a foster dog slipping their collar and running around without identification!