Many of the earlier handwritten letters in the Churchill Archive were written on writing paper which was originally folded (like a booklet). The papers have been flattened and photographed on both sides.
The following letter from Randolph Churchill to Lady Randolph Churchill in 1875 can be used as an example of this: CHAR 28/5/6
So: read the right hand side of the first image on the screen first (this would have been the front page of the letter) and then read the left hand side of the second image (which would have been inside the folded sheet), followed by the opposite side. Then go back to the left hand side of the first image (which, when the letter was folded, would have been at the back).