My theory is that this isn't a pun, but rather a term that's no longer in use. I seem to remember that some early elevator systems had two connected shafts and

My theory is that this isn't a pun, but rather a term that's no longer in use. I seem to remember that some early elevator systems had two connected shafts and two cars, instead of one car and a counterweight like modern elevators. In this arrangement, cars go in a loop around a pulley at the top and bottom, always in the same direction. Thus, one shaft is the "up" shaft and one is the "down" shaft. This image depicts the top floor, so elevators only go down from there. It may have been common practice at the time in France to call the elevator shaft that carries you down to be the descenseur and the one that carries you up to be the ascenseur. Nowadays, ascenseur is used all the time because an elevator car goes both ways.