The d has double contacts as opposed to the single contact. 

1) basic symbol 
(a) part of the symbol consists of one or more than one uppercase letters, said the type of lamp holder. The following said lamp holder of the letter, but, for the lamp holder, they also have a similar meaning. 
B - a bayonet lamp holder; 
BA - bayonet lamp holder, initially used in car lamp; 
BM, miner's lamp with a bayonet lamp holder; 
E - (Edison) screw lamp holder; 
Brought a F -- - out of the contact of the lamp holder; 
Lowercase letters after the letter F different forms of contact, 
Such as: 
A cylindrical pins, said; 
B - pins with a groove; 
C - said special shape pins; 
G - two or more than two protruding contact, 
Such as the 
Pins or terminal; 
K with wire connector socket; 
P -- prefocus lamp holder type; 
R with recessed contacts of the lamp holder; 
S - shell type lamp holder - without protruding parts in the lamp holder to fixed lamp holder; 
SV with tapered at the end of the shell type lamp holder (V); 
T - telephone lamp holder; 
W - said lamp, lamp and lamp holder of electric contact directly through the lead to complete and present on the surface of the lamp end side the glass part (or other insulating materials) is essential for lamp installed in lamp holder. 
To replace the lamp end and sign the same interchangeability requirements of the insulation material of the single lamp holder, can also use symbol; 
Note: some of the earlier symbol shapes similar to form, this will lead to use alphabetic character "W". 
For example: special requirements type of B22 lamp holder BY22d - symbol. 
With G16 GY16 - cannot type basic socket swap G16 type lamp holder. 
Change the method Ⅲ 
In special cases, the symbol of (a) before the part can contain a number, usually is 2. 
The symbol the lamp holder is a two (or more than two) consisting of a single lamp holder similar combination lamp holder. 
2) G13 -- by, for example, has a certain distance between two parallel arrangement mutually G13 type lamp holder combinations of lamp holder (this kind of lamp holder for u-shaped fluorescent lamp). 
Symbol consists of Numbers, (b) part of the lamp holder main dimensions of approximation, it said the unit is mm. The main relations with basic symbol letters as shown below; 
B, BA, BM, K, S, and the Numbers behind the SV the diameter of the shell; 
E Numbers refer to the thread teeth behind the top diameter; 
F at the back of the figure refers to the size of the diameter of the contact or other similar; 
G at the back of the figure refers to the distance between the center of the two pins; - for more than two pins, refers to all the pins in the diameter of the circle center; 
P at the back of the digital representation is used for lateral positioning of the parts of the lamp is the most important dimension; 
R at the back of the digital representation is essential for lamp holder match installed in the lamp holder of the maximum lateral size of the insulating parts; 
T touch the Numbers behind the two pieces of between external width; 
W the Numbers behind the lamp on the end seal leaded glass; (or other insulating materials) sealing parts of the total thickness and width, between the two have a multiplication sign (x); 
BA15b - say, for example, the shell is about 15 mm in diameter car light with a bayonet lamp holder. 
G13 - said the distance between the two pins is about 13 mm double pin lamp holder. 
(c) part of the symbol consists of lower-case letters, it said contact, contact, pins, or amount of flexible joints. The following letter is this kind of usage: 
S - represents a contact; 
D - 2 a contact; 
T - on behalf of the three contacts; 
Q - on behalf of the four contact; 
P -- -- represent the five contact. 
Lamp shell should not be viewed as the contact, it has nothing to do with whether shell current-carrying parts, contact don't are all the same shape. 
E.g. E26d - refers to the two contacts at the bottom of the type of E26 lamp holder; 
G10q - refers to the lamp holder with four contact pins. 
When necessary, the symbol with part (d), it is composed with a hyphen (-) in front of the Numbers, said this part is important for interchangeability of additional components. 
With the number 3, for example, says 3 pin bayonet lamp holder, lamp holder or said the main shape of a certain size. 
For example: B22d - 3 - refers to take three positioning pin type B22 lamp holder. 
PG22-6.35 - the focus ring diameter to avoid 2 mm, about two contact is about 6.35 mm apart prefocus lamp holder. 
Part (e) of the symbol is composed of front with a slash (|) Numbers, it said the approximate value of the total length of the lamp holder, the unit is mm. The length is including the protruding insulating material, but not including the contact or the length of the pins. 
For example: B15 / d19 - refers to the total length of about 19 mm B15d lamp holder. 
SV type (chandeliers) refers to the length of the lamp holder from conical shell diameter 3.5 mm, the distance to the lamp holder openings. To avoid misunderstanding, the length value standard after the hyphen number before the slash. 
Symbol (f) part of the exposure lamp cap shell side or by said belt skirt edge on one side of the outer diameter of digital approximation. Multiplication sign in (f) number before marked (x). The figures show that the skirt does not include the approximation of bell mouth diameter or open the inner diameter, the unit is mm. 
Example: B22d / 25 x 26 - refers to the total length is about 25 mm, skirt outer diameter is 26 mm B22 lamp holder. 
Two other symbols and their meanings 
EP10 / x 11-14 of thread of top 10 mm diameter prefocus type screw base, the total length of about 14 mm, skirt is about 11 mm in diameter. 
B22D - 3 (90? / 135? / 25 x 26 - refers to touch with two piece, the diameter is about 22 mm bayonet lamp holder, it comes with three positioning pin, the radial distribution Angle, respectively for 90? , 135?And 135? ;The total length of about 25 mm, skirt hem about 26 mm in diameter. 
BAY15d / 19 - refers to the belt offset positioning pin, about 15 mm in diameter, there are two contact bayonet lamp holder (automotive), its total length is about 19 mm 
K59d / 80 x 63 - point, with two flexible connector shell lamp is about 59 mm in diameter, the shell is about 80 mm, the total length of skirt is about 63 mm in diameter. 
R17d / 80 x 63 - refers to the localization of the lamp holder in the lamp holder (necessary) insulator of the maximum lateral size of about 17 mm recessed double head, touch light about 10 mm shell height, shell is about 35 mm in diameter. 
SV8.5-8 - refers to the enclosure for the conical shape, its diameter is about 8.5 mm at the end of the shell type lamp holder, the vertebral bodies from the circle diameter 3.5 mm, to measured at the end of the shell opening the shell length is about 8 mm. 
T6.8 - refers to the outer edge of the two contact between the width of about 6.8 mm phone with lamp holder. 
EX10/13 - refers to the leakage distance there is a requirement for additional screw lamp holder, the crest of the diameter of the thread is about 10 mm, the total length is about 13 mm. 
Now commonly used is T8, T5, T4 lamp, commonly used for the previous two years have T10, T12. Then, the "T" what is that mean? What is the difference between T4 and T5? 
"T", on behalf of the "Tube", says it is tubular. 
Behind the Numbers - to put it bluntly, is actually represents the diameter of the tube. Each a "T" is 1/8 of an inch. 
As you know, one inch is equal to 25.4 mm. Then each of the "T" is 25.4 present 8 = 3.175 mm 
T8 tube diameter is 8 * (1/8) * 25.4 = 25.4 mm 
The rest of the figures corresponding to the following: 
T12 38.1 mm in diameter 
T10 diameter of 31.8 mm 
T8 diameter of 25.4 mm 
T5 diameter 16 mm 
T4 diameter of 12.7 mm 
T3.5 diameter of 11.1 mm 
T2 diameter of 6.4 mm