These tags will work with and are compatible with Petsafe cat flaps Ideal for cats/dogs that are not microchipped. Improves flap/feeder reliability in cases where the pet chip doesn't read well. The compatible with Petsafe and compatible with Petsafe Key can be used for an additional pet using them or as a replacement if your pet's Key is lost. Worn on your pet's collar, the Key uses radio-frequency identification (RFID) technology by using a unique code to signal the cat flap to operate as programmed for your pet. The Key is about the size of a quarter and does not require batteries to operate. Program up to 20 Keys to your Door . Key rings included to easily attach to pet's collar. No batteries needed. Simply attach to your cats collar and they're good to go. The tags are used to operate the rfid/microchip cat flaps so that only the cat wearing the tag can enter or exit the house. Each collar tag has a unique code and weighs just 2g Dimensions: 25 x 24 x 5 mm Water-resistant Compatible with the Petsafe cat door PPA19-16145 and PPA44-16427 Package contains: 2x compatible with PetSafe RFID tags