The first series of Ace books began in 1952 with D-01, a western in "tête-bêche" format: Keith Vining's "Too Hot for Hell" backed with Samuel W. Taylor's "The Grinning Gismo". That series continued until D-599, Patricia Libby's "Winged Victory for Nurse Kerry", but the series also included several G and S serial numbers, depending on the price. The D and S did not indicate "Double" (i.e., "tête-bêche") or "Single"; there are D-series titles that are not "tête-bêche", although none of the "tête-bêche" titles have an S serial number. Towards the end of this initial series, the F series began (at a new price), and thereafter there were always several different letter series in publication simultaneously. The D and S prefixes did not appear again after the first series, but the G prefix acquired its own series starting with G-501. Hence the eight earlier G-series titles can be considered part of a different series to the G-series proper. All later series after the first kept independent numbering systems, starting at 1 or 101. The "tête-bêche" format proved attractive to book collectors, and some rare titles in mint condition command prices over $1,000.