Signs in Religious Art

I have seen hundreds of images of centuries old Christian paintings on the Internet and in many of these paintings Catholic popes and other historical figures are depicted as displaying a "two fingered" salute in which the thumb touches the tip of the ring finger, and the index and middle fingers are extended vertically.

Can someone tell me the meaning and origin of this sign?

And where can I read more about it?

Thanks in advance.

mwk222