All light bulb caps and base fittings are labelled with a code consisting of letters and numbers. Before buying a new bulb, look at the existing bulb in the light fitting to ensure you purchase the right size and type of bulb for your specific need.
The first letter of the code tells you the base or cap type of the bulb - for example, Bayonet Cap or Edison Screw Base.
The numbers (in millimeters) tell you the diameter of the base, or the distance between the pins - for example B22, E27 or GU10
Tip: It is important to note whether your light fitting takes screw or bayonet type bulbs, as these are not interchangeable.