Becoming a Clipboard Viewer
A program that is notified of changes in the clipboard contents is called a "clipboard viewer." You get a clipboard viewer with Windows, but you can also write your own clipboard viewer program. Clipboard viewers are notified of changes to the clipboard through messages to the viewer's window procedure.
The Clipboard Viewer Chain
Any number of clipboard viewer applications can be running in Windows at the same time, and they can all be notified of changes to the clipboard. From Windows' perspective, however, there is only one clipboard viewer, which I'll call the "current clipboard viewer." Windows maintains only one window handle to identify the current clipboard viewer, and it sends messages only to that window when ...