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G5
Fluorescent tubes typically have a two pin fitting at each end of the tube. The T5 (16mm diameter) and T4 (12mm diameter) uses the G5 fitting, with 5mm between the two pins.
Both the T8 (26 mm diameter) and the T12 (38mm diameter) both use the G13 cap type, signifying 13mm between the two pins of one end.