The 7-pin B7G tube base was developed in the late 1930s for miniaturized all-glass tubes. The earliest tubes using this base were commercially introduced in 1939, such as the 6J6.
The pins are arranged evenly in a circle of eight positions at 45° increments in a 9.53 mm (3/8") diameter with one pin omitted, which allows the tube to be inserted in only one orientation. The pins are stiff wires protruding through the bottom of the glass envelope and plug directly into the socket. The base is an integral part of the glass envelope. The pinched-off air evacuation nub is at the top of the tube.