A og door is a hinged or spring operated door rather than the more commonly know 'flap' set into a door, wall or window to allow dogs to enter and exit a house on their own without needing a human to open the door. Flaps are hung from the top of the opening and do exactly that - flap. Dog doors generally must be bigger than cat flaps and therefore are often constructed differently; except in rural areas they tend to give access to fenced-in yards or gardens rather than the neighbourhood at large.