This listing is for ONE [1] FACTORY SEALED PetSafe Ultrasonic Bark Control Collar. Per packaging: The PetSafe Ultrasonic Bark Control effectively deters barking when it is worn. When the microphone picks up the sound of your dogs barks, the unit emits an ultrasonic tone that can be heard by your dog, but is silent to most humans. Startled by the high-pitched tone, the dog should stop barking, as it will associate its bark with an unpleasant noise. System Includes: Bark Control Unit Adjustable Nylon Collar [2] Two 3-Vold Lithium CR2032 Batteries Operating Guide SHIPS FREE VIA USPS GROUND ADVANTAGE