This type of behaviour Modification Collar has been around for a long time; they just are not widely understood. These collars can be know as a stabilisation or dominant dog collar. These type of collars need to be used in a specific manner to work with seriously aggressive dogs, dogs that have trouble controlling themselves and run through a range of emotions every second. These need to be used by experienced people and or under the guidance of a trainer or behaviour specialist. They come in sizes that are in 1 inch increments. Fitting needs to be snug , we recommend trainers have a range of sizes so you can test fit to get the exact right size. Please note the length of these is from the clip to the o ring, Not from inside the clips, when measuring your dog. If you are needing some extra length for arresting the collar we do suggest going up a size. These must be fitted high and SNUG and cannot be loose at all. Fitted with our high quality brass snaps and made from very strong nylon webbing. They are very strong and are used to take the aggressive drive out of a dog and they can be used as a back up collar also. Fitting Guide Measure the dog's neck just under the jaw and right behind the ears using a cloth tape measure. The measuring tape should be a measurement, you don’t want the measurement to be loose. There should be no extra room at all between the dog's neck and the tape measure If you have measured your dog’s neck properly and your dog’s neck size is in between two-inch marks round your result down to the nearest inch. For example, if your measurement is 13 1/2 inches than your number is 13 inches. Round up for measurements over half an inch in measurement (13.75" becomes 14"). If you just need a backup collar, simply select the size that matches the number you got. For example, a dog with a 13" or 13.25" neck will need a 13" backup collar.