Martingale training collars come in large, medium, small, and extra small. The picture here is of the medium. Please take note of the snap together buckle on the side of the collar. Once you have set the adjustment on this Martingale collar it will not be necessary to set it again unless your dog is growing. It is not necessary to put this collar over the dog's head. You can leave it attached to the leash and when you are to take your dog out, you can just put it around the dogs neck and snap it together. This type of training collar is becoming more and more popular with dog trainers every day. Thanks for stopping by.