Ace Doubles

American company Ace Books began publishing genre fiction starting in 1952. Initially these were mostly in tête-bêche format with the ends of the two parts meeting in the middle and with a divider between them which functioned as the rear cover of both (the two parts were oriented upside-down with respect to each other in order to effect this), but the company also published some single volumes during the early years. The proportion of singles increased until they stopped producing doubles about 1978. The tête-bêche format was discarded in 1973, but future double novels were continued for a while with both parts oriented the same way, so that the first page of one part followed soon after the last page of the other part.