Among the antecedents of the U.S. Navy's 1920 hull number system was a number series for protected (and a few "unprotected") cruisers, of which more than two-dozen were built or acquired between the mid-1880s and the early 1900s. Twenty-two of these warships received "cruiser numbers", which have informally been abbreviated "C-1" through "C-22". This shortened form was, however, a matter of unofficial convenience and not a part of the Navy's formal hull number system. In 1920-21 the surviving members of the group received new designations and numbers in the Armored Cruiser (CA), Light Cruiser (CL) and Gunboat (PG) series. The old cruiser ("C-") numbers then became extinct.
In addition to the 22 ships numbered in the "cruiser" series, another five protected cruisers did not originally receive numbers, either because they were built earlier or because they were purchased abroad and were not constructed as part of the Navy's shipbuilding program.