Shown here a PHILIPS TL-D, OSRAM LUMILUX L and T8 SYLVANIA Luxline Plus lamp base (G13) or standard fluorescent lamp (tube) lamp base. This cap is called the G13 lamp base. Here is 13 millimeters, the distance between the two lamp pins. The lamp diameter is 26 millimeters.
Sketch lamp base G13 Photo of standard TL-D lamp base G13 cap holder