An MDI application can also store per-document data by using window properties. Per-document data is data specific to the type of document contained in a particular child window. Properties are different from extra space in the window structure in that you need not allocate extra space when registering the window class. A window can have any number of properties. Also, where offsets are used to access the extra space in window structures, properties are referred to by string names. For more information about window properties, see Window Properties.