Working with Device Windows

Microsoft DirectX 8.1 (C++)

This section lists issues that you might encounter when working with device windows in Microsoft® Direct3D® applications.

Direct3D only hooks up the focus windows instead of the device window with the Direct3D message processing function, and only processes the focus window messages. So, the focus window should be the parent of any device window.
For any application, multiple monitor or single monitor, at least one device window must be the focus window. In practical terms, this means that you cannot use a child window of your focus window as the device window in single monitor systems. You must use the parent focus window as both the focus and device windows. For a multiple monitor system, at least one monitor must use the focus window as its device window.