The Microchip Cat Flap Key works with the Microchip Cat Flap and can be used if your pet's microchip ID is not compatible with the flap, for an additional pet using the flap or as a replacement if your pet’s Microchip Cat Flap Key is lost. Worn on your pet's collar, the Microchip Cat Flap Key uses radio-frequency identification (RFID) technology by using a unique code to signal the Microchip Cat Flap to operate as programmed for your pet. The Microchip Cat Flap Key is about the size of a bottle cap and does not require batteries to operate. Each Microchip Cat Flap can program up to 20 Microchip Cat Flap Keys.