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A bayonet mount
A bayonet mount before and after insertion
Early-19th century socket bayonet
Socket of a bayonet

A bayonet mount (mainly as a method of mechanical attachment, as for fitting a lens to a camera) or bayonet connector (for electrical use) is a fastening mechanism consisting of a cylindrical male side with one or more radial pins, and a female receptor with matching L-shaped slot(s) and with spring(s) to keep the two parts locked together. The slots are shaped like a capital letter L with serif (a short upward segment at the end of the horizontal arm); the pin slides into the vertical arm of the L, rotates across the horizontal arm, then is pushed slightly upwards into the short vertical "serif" by the spring; the connector is no longer free to rotate unless pushed down against the spring until the pin is out of the "serif".

The bayonet mount is the standard light bulb fitting in the United Kingdom and in many countries that were members of the British Empire including Pakistan, Australia, India, Sri Lanka, Ireland, and New Zealand, parts of the Middle East and Africa and, historically, France and Greece.



To couple the two parts, the pin(s) on the male are aligned with the slot(s) on the female and the two pushed together. Once the pins reach the bottom of the slot, one or both parts are rotated so that the pin slides along the horizontal arm of the L until it reaches the "serif". The spring then pushes the male connector up into the "serif" to keep the pin locked into place. A practised user can connect them quickly and, unlike screw connectors, they are not subject to cross-threading. To disconnect, the two parts are pushed together to move the pin out of the "serif" while twisting in the opposite direction than for connecting, and then pulling apart.

The strength of the joint comes from the strength of the pins and the L slots, and the spring. To disengage unintentionally, the pins must break, the sleeve into which the connector slides must be distorted or torn enough to free the pins, or the spring must fail and allow the connector to be pushed down and rotate due to, say, vibration.

It is possible to push down the connector and rotate it, but not far enough to engage and lock; it will stay in place temporarily, but accidental disconnection is very likely.

Bayonet electrical connectors are used in the same applications where other connectors are used, to transmit either power or signals. Bayonet connections can be made faster than screw connections, and more securely than push-fit connections; they are more resistant to vibration than both these types. They may be used to connect two cables, or to connect a cable to a connector on the panel of a piece of equipment.

The coupling system is usually made of two bayonet ramps machined on the external side of the receptacle connector and 2 stainless steel studs mounted inside the plug connector’s coupling nut. Several classes of electrical cable connectors, including audio, video, and data cables use bayonet connectors. Examples include BNC, C, and ST connectors. (The BNC connector is not exactly as described in this article, as the male, not female, connector has the slots and spring.)


The first documented use of this type of fitting (without the name "bayonet") may be by Al-Jazari in the 13th century, who used it to mount candles into his candle-clocks.[1] This type of fitting was later used for soldiers who needed to quickly mount bayonets to the ends of their rifles, hence the name.

Light bulbsEdit

LED lamps with GU10 bi-pin twist-lock mount
Compact fluorescent lamp with double contact B22d bayonet mount
Incandescent light bulb with double contact B22d bayonet mount, and corresponding socket with sprung connectors
Preceding bulb mounted in the socket
Incandescent 40 W BA15d bulb

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 St. George Lane Fox-Pitt in the UK and improved upon by the 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. Older railway carriages in the UK also made use of a 3 pin bulb base to discourage theft. 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 infra-red, 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 s or d specifies whether the bulb has single or double contacts.

B15d IEC 60061-1 (7004-11) DIN 49721
BA15d IEC 7004-11 A DIN 49720
BA15s IEC 7004-11 A DIN 49720
BA20d IEC 7004-12 DIN 49730
B22d IEC 60061-1 (7004-10)
GU10 IEC 60061-1 (7004-121)
GZ10 IEC 60061-1 (7004-120)
GU24 Pending (Mar 2007)

While G actually indicates bi-pin, those listed above have a twist-lock, but with parallel pins from the end instead of opposing pins on the side.

These are the available sizes in the UK:[4]

Designation Alternative designation Contacts Dimension, etc.
BA5s 1 5 mm
BA7s 1 7 mm, elongated pins
BAX9s 1 9 mm, radially offset pins
BA9s Miniature bayonet cap (MBC) 1 9 mm
BA15d Small bayonet cap (SBC) 2 15 mm
BAY15d (P21/5W, 1157) 2 15 mm, axially offset pins
BAX15s 1 15 mm, radially offset pins
BA15s (P21W/1156/R5W/R10W) Single centre contact (SCC) 1 15 mm
BA20s 1 20 mm
BA20d 2 20 mm
BA21d 2 21 mm
B21-4 21 mm 4 pin
BA22d Bayonet cap (BC) 2 22 mm
BC-3 Bayonet cap (BC) 2 22 mm 3 bayonet pins
B22d-3 2 22 mm double ended (railway)
BX22d 2 22 mm

Of these, only the BC (BA22d) is widely used in homes. The BA20d (sometimes called a Bosch fitting) was once a common automotive (twin filament) headlamp fitting but has largely been superseded by more modern, higher-rated H-series sockets and is only used for some lower-powered applications such as combined automotive tail and stop lamps.

Other usesEdit

Many cameras with interchangeable lenses use a bayonet lens mount to allow lenses to be changed rapidly and locked accurately in position. Camera lens mounts usually employ stronger flattened tabs rather than pins, though their function is the same.

A bayonet mount is often used to mate a cylinder with a base in cylindrical packaging such as that for CD spindles.

See alsoEdit


Further readingEdit

  • IEC 61184: Bayonet lampholders, International Electrotechnical Commission, 1997. (also: BS EN 61184). Specifies requirements and tests for the B15 and B22 bayonet holders for light bulbs used in some Commonwealth countries

External linksEdit