By default, video devices create a window when an application opens the device, but they do not display it until they receive a window state show command or a play command. Applications can send the MCI_WINDOW message to tell a video device to use an application window instead of the default window to display video. Applications that supply window handles should be prepared to update an invalid rectangle on the window.

Several flags are provided to allow users to manipulate the window. Because MCI_STATUS can be used to obtain the current window handle, programmers might choose to use the standard window APIs instead. The flags are provided to allow applications that use the string interface to perform standard operations.

Support of this message by a device is optional. The parameters and flags for this message vary according to the selected device.