Popular Base Types
A bulb's base or socket is another important part to understanding your bulb needs. There is a wide variety of base types and understanding and recognizing the differences will help ensure you locate the right bulb for your application.
The screw base is the most common for household use and because of this there are many incandescent, halogen, CFL and
LED bulbs that use this socket. Screw bases begin with E (this base style was developed and licensed by Thomas Edison in
1909) and are categorized by their diameter in millimeters. An E12 or "Candelabra" base is 12mm in diameter.
2-Pin & 4-Pin Bases for CFL's
Notice 1) the number of pins; 2) the position of the tabs near the edge of the base; 3) the position of the tabs on the large
square in the center. These tabs are used to key the bulb to a fixture that contains a ballast designed for the wattage of the
bulb. An 18W bulb with a G24q-2 base cannot be plugged in to a fixture designed for 13W bulbs which use the G24q-1 base.