Ace Books began publishing genre fiction starting in 1952. Initially these were mostly in the dos-à-dos (back to back) format, but they also published a few single volumes, in the early years, and that number grew until the doubles stopped appearing in about 1978. The dos-à-dos format was discarded in 1973, but future double novels were continued for a while.
Ace published science fiction, mysteries, and westerns, as well as books not in any of these genres. Collectors of these genres have found the Ace doubles an attractive set of books to collect, because of the unusual appearance of the dos-à-dos format. This is particularly true for the science fiction books, for which several bibliographic references have been written.
Between 1952 and 1968, the books had a letter-series identifier; after that date they were given five-digit numeric serial numbers.
Taken from Wikipedia.