The SureFlap DualScan Microchip Cat Door has a double antenna, which reads a cat"s microchip when it comes into and goes out of the house. This allows you to restrict certain cats to indoor-only, whilst allowing other cats access to the outdoors. If your cat is not microchipped, the DualScan cat door can also be used with a SureFlap RFID Collar Tag. The DualScan cat door can store up to 32 cats and it is compatible with 9, 10 and 15 digit microchips (except those staring with 000, 010 or 020). A SureFlap Cat Door Mounting Adaptor is required for glass installations and one or more SureFlap Cat Door Tunnel Extenders may be required for wall installations. The DualScan cat door is battery powered using 4 AA batteries which last up to a year with normal use. When the batteries start to run low, an indicator light will flash. The DualScan cat door has a 4-way manual lock to enable locking to any combination of in/out and a central magnet on the door prevents it from flapping in the wind. The dimensions of the flap are 142mm (W) x 120mm (H). This is the smallest area your cat will have to pass through.