The bayonet light bulb mount is the standard fitting in many former members of the British Empire including the United Kingdom, Australia, India, Ireland, and New Zealand, as well as parts of the Middle East and Africa (although not Canada, which primarily uses Edison screw sockets along with the United States and Mexico). The standard size is B22d-2, often referred to in the context of lighting as simply BC. Older installations in some other countries, including France and Greece use this base. First developed by George Lane-Fox in the UK and improved upon by the US Brush Electric Company from the late 1870s onward, standard bulbs have two pins on opposite sides of the cap;[2][3] however, some specialized bulbs have three pins (cap designation B22d-3) to prevent use in domestic light fittings. Examples of three-pin bulbs are found in mercury street lamps and fireglow bulbs in some older models of electric radiative heater. Bayonet cap bulbs are also very common worldwide in applications where vibration may loosen screw-mount bulbs, such as automotive lighting and other small indicators, and in many flashlights. In many other countries the Edison screw (E) base is used for lighting.
Some bulbs may have slightly offset lugs to ensure they can be only inserted in one orientation, for example the 1157 automobile tail-light which has two different filaments to act as both a running light and a signal light. In this bulb each filament has a different brightness and is connected to a separate contact on the bottom of the base; the two contacts are symmetrically positioned about the axis of the base, but the pins are offset so that the bulb can only be fitted in the correct orientation. Newer bulbs use a wedge base which can be inserted either way with no issues. Some special-purpose bulbs, such as infrared, have 3 pins 120 degrees apart to prevent them being used in any but the intended socket.
Bayonet bases or caps are often abbreviated to BA, often with a number after. The number refers to the diameter of the base (e.g., BA22 is a 22 mm diameter bayonet cap lamp). BA15, a 15 mm base, can also be referred to as SBC standing for small bayonet cap. The lower-case letter
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