The Octal tube base was introduced in 1935 by RCA for their new metal envelope tubes.
The glass envelope of an octal base tube is cemented into a bakelite or plastic base surrounded by eight metal pins on a 17.45 mm (11⁄16 in) diameter circle in 45° increments. The wire leads from the tube were soldered into the pins, and the seal-off tip through which the air was evacuated from the envelope was protected inside a 7.82 mm (5⁄16 in) diameter keyed post in the center whose key fits an indexing slot in the socket so the tube can only be inserted in one orientation.
Matching plugs were also manufactured that let tube sockets be used as eight-pin electrical connectors.
Octal sockets were also used to mount other components such as electrolytic capacitors. Octal-socketed relays are still common today.