Add another dog to your Free Spirit In-Ground Fence or replace an existing collar with the Free Spirit In-Ground Fence Add-A-Dog Collar. The In-Ground Fence can contain an unlimited number of dogs. The collar automatically works with the fence giving a warning tone/beep and vibration before moving to the shock level you select. Features: Add an unlimited number of dogs to your Free Spirit In-Ground Fence with this Add-A-Dog Collar, or replace your old collar