Light bulb caps and bases are the essential parts that connect a bulb to a light fitting.  A lot of bulbs will have different bases but this should always be advised on the packaging of the bulb itself.  Choosing the right cap/base will make sure the bulb fits properly into your fitting.  Light bulb fittings are standardized globally, but you will find that local preferences and historical standards have led to a variety of different types being available.  

Major Types of Light Bulb Caps and Bases

Light bulb bases are primarily categorized into two main groups: screw-in and bayonet.

1. Screw-in Bases (Edison Screw)

The Edison Screw (ES) bases can be defined by them having a threaded metal cylinder that screws into the socket. These bases use the letter 'E' followed by a number that is a measurement of the diameter of the base in millimeters (mm).

2. Bayonet Bases (Bayonet Cap)

Bayonet Cap (BC) bases feature two or more side-mounted pins that slide into slots which are then twisted to lock the bulb in place.

3. Bi-Pin/Pin Bases 

These bases are identified by the letter 'G' as a prefix and they use two or more pins that push into the socket. They can be found in low-voltage halogen lamps, fluorescent tubes, and modern LED spotlight bulbs. The number following the 'G' will indicate the distance between the pins in millimeters. 

• G4, G6.35, GY6.35: Used for low-voltage 12V capsule smaller style halogen and LED lamps.
• GU10: These have a twist-and-lock bi-pin base and are used with mains voltage (120V/240V) spotlights often used in kitchen and living area recessed lighting. 
• G24, G23: Used in compact fluorescent lamps (CFLs) which are often found in bulkhead light fittings.

Are E27 bulbs being phased out?

E27 bulbs are not being phased out.  They were a common preference for incandescent bulbs and it is this technology that is being phased out, not the E27 cap.  You can still find E27 cap bulbs available in LED.  

What is the difference between E27 and B22?

The difference between E27 and B22 is in the cap and base that they have.  An E27 bulb has a screw in base which looks like a screw whereas a B22 has a bayonet cap base which has two or more pins in the base.  

An E27 bulb needs to be screwed into the compatible socket, whilst a B22 bulb is pushed into the socket and then twisted to secure the pins which then establishes electrical contact.  If you are looking to fit a B22 into an E27 fittings or vice versa you will require a specialised adapter to do this.