While creating a window, the system sends messages to that window's window procedure. The window procedure receives a WM_CREATE message, saying that the window is being created. The window also receives a WM_ADJUSTWINDOWPOS message, specifying the initial size and position of the window being created. This message lets the window procedure adjust the size and position of the window before the window is displayed.

The system also sends other messages while creating a window; the number and order of these messages depend on the class and style of the window and the function used to create it.