To signal specific subscribers on party lines selectively, …

The earliest selective system was the code ringing system, in which each telephone subscriber was assigned a specific ringing cadence, (not to be confused with modern ring tones).

Although various systems were implemented, one that limited the number of coded rings but established a uniform and readily understood format, was to first give the subscriber number as individual digits,

which could be from one to four digits long per exchange, separated by the instructional word "ring" followed by the two digits of the ring code where the first digit indicated the number of long rings,

followed by the second digit indicating the number of short rings.

Thus spoken, for example, as "nine, three; ring two, two" to mean subscriber No. 93 with ringing code 2 long and 2 short, and written as "93R22",

(and if outside the given exchange, then the exchange would be asked for by name before the requested number and ringing code, e.g. "Rockridge nine, three; ring one, two"

i.e. "(On the) Rockridge (exchange), (subscriber No.) nine, three; ring one long, and two short," and written as “Rockridge 93R12”.