The Passport Key‚ works with the Passport‚ Pet Access Smart System‚ and can be used for an additional pet using the Passport or as a replacement if your pet‚ Passport Key is lost. Worn on your pet's collar, the Passport Key uses radio-frequency identification (RFID) technology by using a unique code to signal the Passport to operate as programmed for your pet. The Passport Key is about the size of a quarter and does not require batteries to operate. Each Passport can program up to 20 Passport Keys. Includes one key.