Every main window has a client window, which is the window in which the application displays output and receives mouse and keyboard input from the user. What an application displays in the client window, how it displays it, and how it interprets input to the window are controlled by the client's application-defined window procedure.

An application creates the client window when it creates the frame window. The client window, which is specific to the application, is nearly always created using a private window class (a class registered by the application). Like a frame control, the client window is a child window and is owned by the frame window. This means, for example, that the client window is moved when the frame window moves, is clipped to the frame-window size, and is destroyed when the frame window is destroyed.

The relationship between the frame window and the client window allows the frame window to pass messages between other frame controls and the client window. For example, a client window can send a message to the frame window requesting that the frame window change the window title. The frame window, in turn, sends a message to the title-bar control, telling it to change the title of the window.