Most people choose to have the clear round trancat cat flap installed. its cost effective, looks great and can be upgraded to a magnetic system at any stage should other not-so-welcome cats be coming inside. In its basic standard mode, the cat door has a four way lock easily changed from inside the house. two knobs on either side of the flap are there for you to turn them around until the right locking way is found. you can either full lock the cat door, have the door locked so no cat can come in or locked so no cat can go out, or fully open. the locks best feature is that if your cat is out late at night, set the lock so they can still come in and the cat door locks behind them stopping the flap from opening so they cant go back out all while you peacefully sleep. Most small and toy sized dogs fit through your cat door as well.