stereo images need to be shifted left and right because when they are turned rightside up (they're upside down in the camera) they shift that way.

A transposing frame makes a side-by-side print of the two by allowing you to print one, shift the pair to the side and print the second on the other side of the first, where it belongs. If you fail to transpose them and view the mounted images you get a reverse stereo effect -- stuff that's close looks far, and vice versa.

The transposing frame is used for stereo pairs shot in a large format camera which gives you two images, side-by-side on one piece of film. 35mm cameras shot frames farther apart, with other frames in between, so there are sorting boxes that allow you to cut the frames and automatically transpose them.