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 the contents of the clipboard change.
Clipboard viewer applications have the responsibility of participating in the ”clipboard viewer chain“ so that all running clipboard viewer programs receive the messages that Windows sends to the current clipboard viewer. When a program registers itself as a clipboard viewer, that program becomes the current clipboard viewer. Windows gives that program the window handle of the previous current clipboard viewer, and the program saves this handle. When the program receives a clipboard viewer message, it sends that message to the window procedure of the next program in the clipboard chain.