When Reginald Fessenden won the contract to light the 1893 World’s Fair in Chicago, he opted to use AC (alternating current) power rather than the more popular DC (direct current) power. As a result, Thomas Edison (who ‘invented’ DC power, and who was therefore bitterly disappointed) stubbornly refused to let Reginald use his company’s patented Edison screw light bulbs.
The creation of bi-pin mounts
Because of this, Westinghouse developed a proprietary mount of its own called the bi-pin, or bipin mount (pictured on this page). The bi-pin mount is a common feature of most halogen and LED downlights in particular. They also feature on the ends of many fluorescent tubes. Because bi-pin mounts feature on several different lighting technologies, they are also available in various sizes.
How to identify bi-pin sizes
To find the right sized base for your light fitting, first look for the letter G, followed by two numbers.
These numbers refer to the base of the light bulb's diameter - in millimetres.
If the light bulb features he letter ‘q’, this indicates that the bulb is designed for a quad-pin mount (i.e. it has four pins rather than two). Smaller bi-pin mounts also exist - these are usually referred to as mini bi-pin mounts.