Screw type bulb socket with a diameter of 0.4 inch (10 mm) to fit fairy bulbs used in flashlights.
It is also used as school materials.
Pilot lamps are common but some E-12 models are similar to the standard.
(The E-12 model is common with the naked bulbs used in household fluorescent fixtures.
Also the E-11 model is slightly thicker for halogen lamps with mirrors.