Light Bulb Bases Guide Part 1: Screw Base Light Bulbs

There are two main types of light bulb bases: screw bases and pin bases. Each works in a different way to connect the light bulb to the electrical voltage in order to light the bulb. In this article, we will discuss screw bases. To learn about pin bases, click here.

What are Screw Bases?

Screw bases are the most common light bulb base. In an incandescent or halogen screw base light bulb, two contact wires connect the filament to the base of the light bulb, where the electrical voltage connects to the light bulb. The contact wires are soldered to the base metal at two different points on the base in order to keep them affixed to the base metal.

If the contact wires were not permanently affixed to the base metal, electrical current would not be able to continuously flow through the wires to energize the filament into producing light. One of the contact wires is soldered on the side of the base, and the other is soldered at the bottom. Therefore, the light bulb makes electrical contact at two points on the base.

Did you know that LEDs are 80-90% more energy-efficient than traditional bulbs? Learn how you can replace your old Lighting to cut energy costs.

Common Screw Base Styles

Screw bases come in several sizes, and they are referred to by both a descriptive name and size.


Unlike the light bulb shapes, the number in the light bulb base measures the light bulb base diameter in millimeters. The candelabra E12 has a diameter of 12 millimeters.

The “E” in the light bulb size refers to Edison, the inventor of the screw base light bulb, and screw base light bulbs are sometimes called “Edison light bulbs.” Candelabra bases are mostly used in smaller, decorative incandescent, CFL, and CCFL light bulbs.

The intermediate E17 base is not very common. It is found on smaller light bulb shapes such as S11, R12, and T6 light bulbs.

The medium E26 base is the most common light bulb base. It is used in most incandescent, halogen, CFL, CCFL, and HID light bulbs. The mogul E39 base is found in larger sized light bulbs, such as high wattage HID.

The mogul pre-focused (MPF) base is similar to the screw base in that it connects the filament to the base with two soldered contact wires; however, MPF bases do not screw into the socket. Rather, the base metal has a smooth edge, and the base is pushed into the socket and then turned to lock it into place. The light bulb base and socket are manufactured in a way that predetermines the light bulb’s position in the socket. No matter how the end user places the light bulb base into the socket, it will always turn and lock into the same position every time, creating a fail-safe for the light bulb’s operation. Light bulbs with special coatings designed to focus light downward may use this type of base so that the light bulb will always be installed in the correct position to distribute and focus light properly.

The bayonet base is a smaller fail-safe light bulb base. Similar to the mogul pre-focused base, it has a smooth base metal that is placed into the socket with a push fit to ensure the light bulb’s proper position in the socket. However, the double contact bayonet (DCB) has two contact points at the bottom of the base that protrude from the base to make contact electrical contact. Bayonet bases are very rare in the U.S. They are more prevalent in Europe, Australia, and New Zealand, and they are primarily used in incandescent and CFL light bulbs.

The wedge base is found in incandescent T3.25 light bulbs. Wedge bases do not have any base metal or solder. Rather, they are made by tapering the glass of the bulb in a manner that seals the light bulb. Two wires at the base of the glass act as the electrical voltage contact points for the wedge base.


Learn More

We offer a number of resources to help you navigate the world of LED lighting including videos, case studies, and a lighting blog. However, you can also contact us or find a sales rep in your area here!


Related Links:

Light Bulb Shapes Guide

Lighting Bases Guide Part 2: Pin Base Light Bulbs

Lighting Bases Guide Part 3: 3-Way Base Light Bulbs 

Lighting Bases Guide Part 4: Lock-Out Base Light Bulbs