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.