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Reference designator

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A reference designator unambiguously identifies the location of a component within an electrical schematic or on a printed circuit board. The reference designator usually consists of one or two letters followed by a number, e.g. C3, D1, R4, U15. The number is sometimes followed by a letter, indicating that components are grouped or matched with each other, e.g. R17A, R17B. The IEEE 315 standard contains a list of Class Designation Letters to use for electrical and electronic assemblies. For example, the letter R is a reference prefix for the resistors of an assembly, C for capacitors, K for relays.


IEEE 200-1975 or "Standard Reference Designations for Electrical and Electronics Parts and Equipments" is a standard that was used to define referencing naming systems for collections of electronic equipment. IEEE 200 was ratified in 1975. The IEEE renewed the standard in the 1990s, but withdrew it from active support shortly thereafter. This document also has an ANSI document number, ANSI Y32.16-1975.

This standard codified information from, among other sources, a United States military standard MIL-STD-16 which dates back to at least the 1950s in American industry.

To replace IEEE 200–1975, ASME, a standards body for mechanical engineers, initiated the new standard ASME Y14.44-2008. This standard, along with IEEE 315–1975, provide the electrical designer with guidance on how to properly reference and annotate everything from a single circuit board to a collection of complete enclosures.


ASME Y14.44-2008[1] and IEEE 315-1975[2] define how to reference and annotate components of electronic devices.

It breaks down a system into units, and then any number of sub-assemblies. The unit is the highest level of demarcation in a system and is always a numeral. Subsequent demarcation are called assemblies and always have the Class Letter "A" as a prefix following by a sequential number starting with 1. Any number of sub-assemblies may be defined until finally reaching the component. Note that IEEE-315-1975[2] defines separate class designation letters for separable assemblies (class designation 'A') and inseparable assemblies (class designation 'U'). Inseparable assemblies—i.e., "items which are ordinarily replaced as a single item of supply"[2]—are typically treated as components in this referencing scheme.


  • 1A12A2R3 - Unit 1, Assembly 12, Sub-assembly 2, Resistor 3
  • 1A12A2U3 - Unit 1, Assembly 12, Sub-assembly 2, Inseparable Assembly 3

Especially valuable is the method of referencing and annotating cables plus their connectors within and outside assemblies. Examples:

  • 1A1A44J5 - Unit 1, Assembly 1, Sub-Assembly 44, Jack 5 (J5 is a connector on a box referenced as A44)
  • 1A1A45J333 - Unit 1, Assembly 1, Sub-Assembly 45, Jack 333 (J333 is a connector on a box referenced as A45)

A cable connecting these two might be:

  • 1A1W35 - In the assembly A1 is a cable called W35.

Connectors on this cable would be designated:

  • 1A1W35P1
  • 1A1W35P2

ASME Y14.44-2008 continues the convention of Plug P and Jack J when assigning references for electrical connectors in assemblies where a J (or jack) is the more fixed and P (or plug) is the less fixed of a connector pair, without regard to the gender of the connector contacts.

The construction of reference designators is covered by IEEE 200-1975/ANSI Y32.16-1975[3] (replaced by ASME Y14.44-2008[1]) and IEEE-315-1975.[2]


The table below lists designators commonly used, and does not necessarily comply with standards. For modern use, designators are often simplified towards shorter designators, because it requires less space on silkscreens.

Designator Component type Modern use
A Separable assembly or sub-assembly (e.g. printed circuit assembly)
AT Attenuator or isolator
BR Bridge rectifier (four diodes in a package) often changed to "D" for diode
BT Battery often shortened to "B"
C Capacitor
CB Circuit Breaker
CN Capacitor network may be simplified to "C" for capacitor
D, CR Diode (all types, including LED), thyristor "D" is preferred for various types of diodes
DL Delay line
DS Display, general light source, lamp, signal light
F Fuse
FB Ferrite bead sometimes changed to "L" for inductor, though "E" was used in the currently inactive standard IEEE 315 (see Clause 22.4).
FD Fiducial
FL Filter
G Generator or oscillator
GL Graphical logo
GN General network
H Hardware, e.g., screws, nuts, washers sometimes hardware is expanded to "HW"
HY Circulator or directional coupler
IR Infrared diode often changed to "D" for diode
J Jack (least-movable connector of a connector pair), jack connector (connector may have "male" pin contacts and/or "female" socket contacts) all types of connectors, including pin headers.
JP Jumper (link)
K Relay or contactor
L Inductor or coil or ferrite bead
LS Loudspeaker or buzzer
M Motor
MK Microphone
MP Mechanical part (including screws and fasteners)
OP Opto-isolator often changed to "U" for IC
P Plug (most-movable connector of a connector pair), plug connector (connector may have "male" pin contacts and/or "female" socket contacts)
PS Power supply
Q Transistor (all types)
R Resistor
RN Resistor network sometimes simplified to "R" for resistor, or "N" for network
RT Thermistor sometimes simplified to "R" for resistor
RV Varistor, variable resistor
S Switch (all types, including buttons) sometimes SW is erroneously used
SA Spark Arrester
T Transformer sometimes changed to "L" for inductor
TC Thermocouple
TP Test point
TUN Tuner
U, IC Integrated circuit (IC) shorter "U" (unit) is preferred instead of "IC"
V Vacuum tube
VR Voltage regulator (voltage reference), or variable resistor (potentiometer / trimmer / rheostat) voltage regulators are often "U" for IC, pots and trimmers often "R" for resistor
X Socket connector for another item not P or J, paired with the letter symbol for that item (XV for vacuum tube socket, XF for fuse holder, XA for printed circuit assembly connector, XU for integrated circuit connector, XDS for light socket, etc.)
X, XTAL, Y Crystal, ceramic resonator, powered oscillator

Other designators[edit]

See also[edit]


  1. ^ a b Reference Designations for Electrical and Electronics Parts and Equipment: ASME Y14.44-2008 (Replaced IEEE 200-1975). ASME, Fairfield, NJ. 2008.
  2. ^ a b c d IEEE (1975), "22. Class Designation Letters", IEEE Std 315-1975: Graphic Symbols for Electrical and Electronics Diagrams (Including Reference Designation Letters) (Reaffirmed 1993), IEEE and ANSI, New York, NY
  3. ^ Standard Reference Designations for Electrical and Electronics Parts and Equipments: IEEE 200-1975 (Reaffirmed 1988): Section (2). IEEE and ANSI, New York, NY. 1975.

Further reading[edit]