Where is the backdoor slider pitch located?

The term "Back Door" could be used for both a slider, and a curve ball. It refers to a Baseball that was thrown outside of the strike zone, misses the plate but, is caught behind the plate, and called a strike. Technically it should be called a "ball" because it never crosses the plate but, because it is caught behind the plate and appears to be a strike to the umpire; It is said it came in through the "back Door"