Light Bulb Specs, Lighting 101

All About Light Bulb Caps and Bases

light bulbs caps bases

The cap or base of a light bulb joins the bulb to the lamp socket in order to provide an electrical connection. Your lamp or fixture’s socket will dictate which cap or base a light bulb needs in order to fit, but there are a number of different base types and sizes to choose from, and it’s not always easy to determine which type is right for your fixture. To add to the confusion, different fixtures often feature different types of sockets.

Purchasing a bulb with the wrong cap or base is a common mistake. There are a wide variety of caps and bases available, and it can be difficult to know which one is right for your fixture, especially if its socket is not a standard size. To help you find the right cap or base for your fixture, we’ve created a guide to the most common light bulb caps and bases. If you are still unsure after reviewing our guide we would be happy to try and help you find the right bulb!

When you’re shopping for new bulbs or upgrading to energy efficient lamps, these are some of the caps and bases you’re most likely to encounter:

  • Bayonet (twist and lock bases)
  • Screw (also known as “Edison”)
  • Pin and push fit

Each light bulb has a different code to indicate which type of base it has, as well as to provide some detail about its specifications. Let’s take a closer look at these codes and what they mean.

Understanding The Coding System

There may be several codes on a package to help you understand the type of light bulb you are buying, but these codes can often add to the confusion of selecting a new bulb.

Base codes follow a letter-number-letter format:

  • The first letter(s) indicate the type of base or cap, such as Bayonet or Edison.
  • The numbers indicate the diameter of the base or the distance between the pins in millimetres.
  • The last letter is optional. It indicates the number of pins or contacts the base has and can usually be found on architectural and striplights.

For example, a GU10 is a multi-pin base (in this case a bi-pin) with a universal (U) beveled ceramic base and a distance of 10mm between the pins.

Light Bulb Base & Socket Shape Types

Number of Pins or Contacts (optional)

BBayonet CollarsSingle
EEdison ScrewdDouble or Bi-pin
FSingle PintTriple pin, tri-pin, or 3 pin
GMultiple Pin (Bi-pin)qQuadruple pin, quad pin, or 4 pin
KCable Connections
RRecessed Contact(s)
TTelephone Slide
WWedge Base
XSpecial Type

Let’s explore the most common base types in more depth:

Bayonet (Twist & Lock Bases)

There are two main types of Bayonet bases: Bayonet caps and GU10.

Bayonet Cap

Named for a fitting design that utilizes the same push and twist action used by soldiers to mount bayonets to their rifles, a bayonet cap is the most commonly used base in the UK. When you push and twist a bayonet base into the socket, the pins retract and expand into slots inside the socket, securing the bulb in place. A key benefit of this style of cap is that it does not rely on threads to stay in place, so it cannot be stripped or accidentally removed.

bayonet cap b22

The standard size for bayonet caps has a smooth metallic base and is 22mm in diameter, with two moveable pins on either side, but bayonet caps are available in a range of large and small sizes.


The letter “G” indicates a bi-pin or two pin base, but because GU10s have a twist and lock mechanism, they are classified as a type of bayonet base. GU10s have two pins that are 10mm apart and extend 7mm from the base. They are most commonly used in:

  • Track style spot lights
  • Kitchens
  • Bathrooms
  • Retail showrooms
  • Directional down lighting
  • Recessed lighting

gu10 cap

Screw Bases

The Edison screw base is named for inventor and innovator Thomas Edison. An Edison screw base is a threaded metal base that holds the bulb securely in place. The most commonly used size is the E27, often found in domestic settings and designed to fit in standard light fixtures.

Smaller Edison screw bases, such as the E14 (small Edison screw, or SES), E12 (candelabra Edison screw, or CES), and E10 (miniature Edison screw, or MES), are primarily used in bulbs designed for decorative lighting, such as chandeliers.

edison screw cap e27

small edison screw cap e14

Pin and Push-fit Bases

Push-fit bases are exactly what they sound like – they are pushed into the fittings rather than screwed or twisted. Typically these bases will have pins, but the number of pins varies, as does their shape and length. The pins range from looped wires to short metal stumps to thin prongs. The most common is the bi-pin, which is usually found on incandescent, halogen or fluorescent lamps.

pin push fit cap

Unlike bayonet and Edison screw bases, pin and push-fit bases do not have integrated control gear. Integrated control gear allows the light bulb to regulate itself. Light bulbs that don’t have integrated control gear require an external source of regulation, such as a ballast, transformer or driver.


Most halogen spotlights are push-fit pin bases with specialized shapes to prevent them from being used in an incorrect socket. For instance, the GU10 is a bi-pin cap with pins 10mm apart and a beveled ceramic base. The “U” in GU stands for universal, so it will safely fit in both GU10 or GX10 fixtures. In comparison, GX10 is a bi-pin with its pins 10mm apart, but it has a square ceramic base. Because of its square base shape, it is only capable of fitting in a GX10 fixture.

Square or Rectangle Fittings

Compact fluorescent lamps (CFLs) and LEDs are both available in bayonet and Edison screw bases, but they are also available as push-fit pin bases. They are usually square or rectangular fittings with two to four pins.

Tubes and Striplights

Fluorescent tubes and LED striplights usually have two pins at either end of the tube. Unlike standard striplights, which can be bi-pin or have an S15s fitting at either end, architectural strip lamps are available with two different kinds of fittings: one with an S14s connector at either end of the lamp and one with a central S14d connector that has two contact points, as indicated by the lowercase “d”.

Wedge Base

The wedge base is similar to a bi-pin base, but the two “pins” are malleable wires that extend from the bulb to the external part of the base where they make contact with the socket. They have to be inserted and removed with force, and are typically found on fairy lights.

Find the Right Cap

The most commonly used bulbs for around the home are:

  • B22 bayonet cap
  • E27 screw cap
  • E14 small screw cap
  • GU10 spotlight (mains voltage)
  • GX53 (low voltage transformer)

To find the right cap for you incandescent, halogen, CFL, and LED bulbs, view our complete listing of caps and bases.

Building a Solid Knowledge Base

It’s disappointing to get home after buying a light bulb only to discover it’s the wrong size or type of base. Nobody wants the hassle of going back to the shop and making a return. Understanding what to look for with regards to terms and codes on bulb packaging can help save you time, effort, and even money in the long run.

If you’re interested in learning more about light bulbs, check out our tools and resources.

If you are still unable to locate the cap type for your particular Light Bulb then please call us on: 01869 362222, email us or for instant expert advice use our “Live Support Online” facility.


About Sarah Levison

Sarah is a lighting expert at The Lightbulb Company with a background in hospitality and a keen awareness of the important role lighting plays in our everyday lives. She has worn many hats with us over the years, developing an extensive knowledge of lighting and the lighting industry. She is passionate about working with customers, understanding their needs, and providing them with the best solutions.

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