Bi-pin lamp base

The bi-pin base was invented by Reginald Fessenden for the 1893 World's Fair in Chicago.

[1][non-primary source needed] The suffix after the G indicates the pin spread; the G dates to the use of Glass for the original bulbs.

GU usually also indicates that the lamp provides a mechanism for physical support by the luminaire: in some cases, each pin has a short section of larger diameter at the end (sometimes described as a "peg" rather than a "pin"[2]); the socket allows the bulb to lock into place by twisting it; in others, the base of the lamp has a groove which can be held by a spring or clip.

[3] A lowercase "q" at the end of the designation indicates that it is a quad-pin base, with two bi-pin pairs.

These are used with compact fluorescent tubes that plug into a light fixture that has a permanent ballast.