Type of Light Bulb base & Socket:Part-1


  • A lamp holder is the device which holds a light bulb or lamp. It’s important that the lamp holder is compatible with the type of bulb we want to use, with the wrong base selection will not fit in the lamp or fixture.
  • Light bulb sockets are normally defined by a letter-number-letter (Third Letter is optional).
  • The first letter indicates the shape of the base.
  • The Second numbers indicates either the width of the base or the distance between the pins.
  • The Third Letter indicates the number of pins or contacts on the lamp. The numbers are normally in millimeters.

Type of Sockets:

There are different types of Socket available in market

  1. Bayonet Cap Base (B)
  2. Edison Screw Cap Base (E)
  3. Single Pin Type Base (F)
  4. Bi Pin or Multi Pin Type (G)
  5. Cable Connections (K)
  6. Pre- focused Light Base (P)
  7. Recessed Contact Base (R)
  8. Flange Base Light Bulb Base (F or T)
  9. Slide Base (S)
  10. Wedge Base (W)
  11. Special Type Light bulb Base (X)

(1)  Bayonet Cap (B / BC / SBC):


  • Type of Connection: “push and twist” action
  • Pin Configuration: (Bxx or BCxx):
  • The First letter indicates the shape or Style of the base.
  • The Second number indicates the width of the base (normally in millimeters).
  • Example:
  • The “B” refers to the style of Base which is a Bayonet, and the 22 means it has a 22 mm base width.
  • Application: All Regular type of Bulbs including specialist low voltage halogen lamps

(2) Edison Screw Cap (E / ES):

  • Named as inventor Thomas Edison, the Edison Screw or “ES” lamp fitting is used in a vast range of applications.


  • Type of Connection: “Screw” Action
  • Pin Configuration: (Exx or ESxx):
  • The First letter indicates the shape or Style of the base.
  • The Second number indicates the width of the base (normally in millimeters).
  • Example: E26 base.
  • The “E” refers to the style which is an Edison screw-in, and the 26 means it has a 26 mm base width.
  • Application: Large chandeliers Some Decorative Lamps.
  • The most commonly used screw fittings are shown in the table below. Other, less frequently used, sizes include E11, E17 and E26.
Designation Diameter Name Abbreviation
E5 5mm Lilliput Edison Screw LES
E10 10mm Miniature Edison Screw MES
E12 12mm Candelabra Edison Screw CES
E14 14mm Small Edison Screw SES
E17 17mm Intermediate Edison Screw Base IES
E27 27mm Medium Edison Screw ES
E39 39mm Mogul or Giant Edison Screw Base GES
E40 40mm Giant Edison Screw GES

(3) Bi Pin or Multiple Pin Type Base (G)


  • The “G” type of lamp base is used to as a “pinned” base. This may be several Numbers of different Types of pins.
  • Type of Connection: “Pin” Type
  • Pin Configuration: (G(U,X,Y,Z) -xx-x-x):
  • It is indicate by Letter (or Letters)-Numbers-Optional letter- Letter:
  • The first letters includes “G” which is followed by a Second (optional Letter) Letter U, X, Y or Z.
  • The second letter (U, X, Y,Z) represents the configuration of the base, which determines the diameter and shape of the pins. For example, pins may be rounded, square, grooved, fat or slender.
  • Without any Optional Letter = Base length is 8.25mm and Pin Diameter is 0.7mm
  • For Y=Base length is >6mm and Pin Diameter is 0.7mm
  • For X= Base length is <7.5mm and Pin Diameter is 1mm
  • For U= Base length is >6mm and Pin Diameter is 1mm With One or Two Grip channel.
  • For Z=P Base length is >6mm and Pin Diameter is 1mm without Grip Channel.
  • The Third number indicates the Distance in millimeters between the centers of each pin.
  • The Forth (optional Letter) Letter indicate the number of pins, no letter means 2 pins. d for double Pin ,s for Single pin, t for Triple pin or 3 pins and q for Quadruple pin or 4 pins.
  • The Fifth Letter indicates position of Dowel (Slot) in Socket
  • Examples: GU10 base type, GU24q 2Pin base type, G13d 3pin.
  • For GU24q 2 Socket, U=Pin length is>6mm with diameter 1mm with Grip channel, No of Pin is 4 no’s and Dowel is on Left Side
  • Application: Halogen, Compact Fluorescent ,LED Types Bulbs


Difference between G24q-1, G24q-2, G24q-3 Type Socket

  • Here G=Type/ Shape of Socket
  • 24=Distance between each Pin
  • q= Quadruple (Four) Pin.
  • 1=Indicate Dowel (Slot) Position ,1=Middle, 2=Left, 3=Right Position

Difference between Gx24, G24 Type Socket

  • In Gx24 The Base is smaller while in G24 Base is larger.


Type Pin to pin distance Pin Diameter Typical Bulbs that use this base
G4 4mm 0.65-0.75mm MR11 and other small halogens of 5/10/20 watt and 6/12 volt
GU4 4mm 0.95-1.05mm
GY4 4mm 0.65-0.75mm
GZ4 4mm 0.95-1.05mm
G5 5mm T4 and T5 fluorescent tubes
G5.3 GU5.3 GX5.3 GY5.3 5.33 mm 1.47-1.65mm MR16 and other small bulbs typically using 12/24 volts
G6.35 GX6.35 GY6.35 6.35 mm 0.95-1.3mm
G8 8mm
G9 9mm
GU10 10mm bulbs using twist-lock bi-pin base
G13 12.7mm T8,T10 & T12 fluorescent tubes
G23 23mm 2mm
GU24 24mm 2 and 4 pin bases with center key
GX53 53mm used with puck shaped lamps typically twist-lock
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