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B15 & B22: What are Bayonet bases?

The bases B15 and B22 are available in many subspecies. The B stands for bayonet base, the numbers behind it for the diameter of the base in millimeters. The areas of application range from signal lamps to machines and marine applications. Lamps with bayonet bases are particularly widespread in the field of automotive lighting.

Lamps with bayonet bases


Explanation: the bayonet base

The “B” in the name stands for the term bayonet base. This has nothing to do with the originalbayonet, the French weapon. But the name is no coincidence, as it refers to the bayonet lock. This works in a similar way to the bayonet, which was put on the rifle as a stabbing weapon, and is defined as a connection between tubular parts made by turning, which can be easily detached again. Thus, a bayonet base is a socket with a bayonet lock for lamps. Another name for the bayonet base is Swan socket, after the inventor Joseph Wilson Swan, who invented the incandescent lamp together with Edison.


If the letter “B” is followed by another letter, it provides further information about the specification of the lamp. The letter “Y” e.g. indicates that the light output is yellow. If this call sign is missing, the light will be white. Very often, the specification “A” can be found. It refers to the automotive sector as the suitable area of application (“A” stands for automotive. The number behind the letter refers to the diameter of the base, measured in millimeters. The contact pins, however, do not count. There is often a small letter found behind the number, e.g. with a B22d base. This refers to the amount of base contacts. The most frequently used one is “d” for double, hence a socket with two base contacts. The letter “s” is short for single, meaning that the lamp contains only one base contact. Less popular is a “t” (triple, three contacts), “q” (quadruple, four contacts) or “p” (penta, five contacts)


B15 & B22: Application areas

Since with a bayonet lock the loosening of the connection is practically avoided even in the event of mechanical vibrations, this type of lamp is particularly suitable for use in the automotive sector. There are also different base names depending on the type of vehicle lighting: 

  • BA7s: Interior lighting of motor vehicles, e.g. as the backlight of the speedometer
  • BA9s: Side lights
  • BA15s: Indicator-, stop-, reversing-, rear fog-, daytime running- and licence plate lights
  • BAU15s: indicator (coloured lights)
  • BAY15d: two pins, offset in height, for brake- and rear light
  • BA20s: Vehicle lighting, especially for railways
  • BA20d: Signal lamps


In addition to the automotive sector, device lighting, machines, aircraft and ships are also common areas of application for lamps with bayonet bases. For example, BA15d lamps are used as car lamps, but are also suitable as spotlights for campers, boats and bicycles. Thanks to their small size, lamps with a BA15d or B15d base can often be found as a light source in sewing machines.



In some countries, lamps with bajonet bases are also used for applications in private households, e.g. in France, the United Kingdom, Ireland or USA. The most common base types there are B15d and B22. 


How do I switch to LED?

B15d and B22d lamps are operated directly on the main voltage. Switching to efficient LED lighting is therefore very easy - a transformer is not required.