The Petsafe PBC-302 Bark Control Training System features 6 levels of correction and is one of the smallest and lightest unit available at only 1.5 oz. When your dog barks, a harmless static correction is emitted to correct the behavior. It is a sufficient deterrent to excessive barking. Adjustable collar for neck sizes 6"-28". Also comes with a training video. This collar helps your dog learn the difference between good barking (for protection) and bad barking (due to boredom or over-excitement). The collar picks up the vibrations from your dog’s vocal chords as it barks. The vibrations cause the collar to send an electronic signal (a static shock) to your dog. Your dog will quickly learn that barking causes the signals and that not barking is the way to avoid the signals.