Transposition is a step in the development process of a stereophoto to make the images suitable for viewing with a stereoscope. When shooting a subject, the image is reversed through the camera lens. The image is “upside down”. With a normal photo this is not a problem, but with a stereo photo it means that if the negative is turned to the normal position, the left and right images of a stereopair are swapped. When such a developed stereo photo is viewed in a stereoscope, it gives an unnatural image because the foreground and background are interchanged. This is called pseudoscopy. To prevent this, the image must be transposed.
When developing a stereo negative, the left image is printed on the right and the right image is printed on the left. In a process without a negative, like autochrome, the negative with the exposed image has to be cut through and the two images are exchanged and reattached to each other.