There are a large types of lighting bases and sockets. You need to ensure that the right base fits into your light fitting.
Bayonet Cap Light Bases
This is the common push and twist base
, which is exists on most regular light bulbs. Its diameter is 22mm ands has two locating lugs.
Edison Screw Cap Light Base
The Edison bases are screw-ins, with the most popular socket (E27) having a 27mm diameter. The E14 Edison is commonly used for chandeliers and appliances.
Halogen Capsule Bases
Halogen capsule bases are designated by the number of millimeters between the pins. For example the G4 has pins 4mm apart.
The most common downlights are push fit (GU4, GU5.3 or GU10). They come in various forms, including halogen and LED.
Fluoresent Tube Bases
Fluorescent tubes normally have a two-pin fitting at the ends of the tube.