What prefixes should be used on reference designators for components of various types? I think we can all agree that "R##" refers to a resistor, "C##" refers to a capacitor, and "L##" refers to an inductor. Beyond that, it appears to be a mishmash of conventions.

I've seen both "IC" and "U" used for chips. "Q", "T", and "M" have been used for transistors and MOSFETs. Ordinarily, "D##" is used for diodes, but LEDs and Zeners get special treatment on some boards.

What standards are available for reference, and who endorses them? JEDEC, ISO, IEEE, and other standardization bodies are welcome references (though I'd prefer an inexpensive standard), but I'm also curious to see what's used at various companies other than my own.

Edit: What I'd really like to see is a list in an answer here which complies with the standard (even if it's just a quote).

  • \$\begingroup\$ The nice thing about standards is that there are so many to choose from. (In other words, look at a variety of schematics, and choose something that makes sense to you.) \$\endgroup\$ Commented Jul 25, 2018 at 3:35

4 Answers 4


There are actually standards to address this, IEC 60617 (also known as British Standard BS 3939), ANSI standard Y32 (also known as IEEE Std 315), Australian Standard AS 1102

Below is a table of some common markings from this link to an old revision of a Wikipedia article

  • Designator Component Type
  • AT Attenuator
  • BR Bridge rectifier
  • BT Battery
  • C Capacitor
  • CN Capacitor network
  • D Diode (including zeners, thyristors and LEDs)
  • DL Delay line
  • DS Display
  • F Fuse
  • FB or FEB Ferrite bead
  • FD Fiducial
  • J Jack connector (female)
  • JP Link (Jumper)
  • K Relay
  • L Inductor
  • LS Loudspeaker or buzzer
  • M Motor
  • MK Microphone
  • MP Mechanical part (including screws and fasteners)
  • P Plug connector (male)
  • PS Power supply
  • Q Transistor (all types)
  • R Resistor
  • RN Resistor network
  • RT Thermistor
  • RV Varistor
  • S Switch (all types, including push-buttons)
  • T Transformer
  • TC Thermocouple
  • TUN Tuner
  • TP Test point
  • U Integrated circuit
  • V Vacuum Tube
  • VR Variable Resistor (potentiometer or rheostat)
  • X Transducer not matching any other category
  • Y Crystal or oscillator
  • Z Zener Diode

Component name abbreviations widely used in industry:

  • AE: aerial, antenna
  • B: battery
  • BR: bridge rectifier
  • C: capacitor
  • CRT:cathode ray tube
  • D or CR: diode
  • DSP:digital signal processor
  • F: fuse
  • FET:field effect transistor
  • GDT: gas discharge tube
  • IC: integrated circuit
  • J: wire link ("jumper")
  • JFET: junction gate field-effect transistor
  • L: inductor
  • LCD:Liquid crystal display
  • LDR: light dependent resistor
  • LED: light emitting diode
  • LS: speaker
  • M: motor
  • MCB: circuit breaker
  • Mic: microphone
  • MOSFET:Metal oxide semiconductor field effect transistor
  • Ne: neon lamp
  • OP: Operational Amplifier
  • PCB: printed circuit board
  • PU: pickup
  • Q: transistor
  • R: resistor
  • RLA: RY: relay
  • SCR: silicon controlled rectifier
  • SW: switch
  • T: transformer
  • TFT:thin film transistor(display)
  • TH: thermistor
  • TP: test point
  • Tr: transistor
  • U: integrated circuit
  • V: valve (tube)
  • VC: variable capacitor
  • VFD: vacuum fluorescent display
  • VLSI:very large scale integration
  • VR: variable resistor
  • X: crystal, ceramic resonator
  • XMER: transformer
  • XTAL: crystal
  • Z or ZD: Zener diode
  • \$\begingroup\$ The Wikipedia article has both a table and a list. The table says "The table below lists designators commonly used, and does not comply with the standard." The list says "Component name abbreviations widely used in industry." But, upvoted for the list standards and the Wikipedia references to those standards (and others). \$\endgroup\$ Commented Feb 17, 2011 at 19:31
  • 1
    \$\begingroup\$ Quick note: AS 1102 / IEC 60617 are for graphical symbols. Letter designators are from AS 3702 / IEC 60750. See electronics.stackexchange.com/questions/200428/…. \$\endgroup\$ Commented Nov 12, 2015 at 11:45
  • 1
    \$\begingroup\$ Quick note: IEC60750 is redrawn and the newer ISO/IEC 81346:2009 includes reference designators \$\endgroup\$
    – oldtimer
    Commented Jun 6, 2019 at 6:13

I've used the canonical R, C, D, Q, U scheme at both career stops thus far.

My first stop would differentiate between 'male' and 'female' connectors by using J for 'jacks' (i.e. female) and P for 'pins' (i.e. male).

I currently use RT for NTC/PTC thermistors and RV for MOVs. Odd that the powers-that-be only opted for such a narrow amount of reference designator innovation.

  • \$\begingroup\$ What makes that scheme canonical? \$\endgroup\$ Commented Feb 17, 2011 at 5:02
  • \$\begingroup\$ Sounds about the same as what I've seen. \$\endgroup\$
    – Nick T
    Commented Feb 17, 2011 at 5:02
  • 2
    \$\begingroup\$ I suppose since the overwhelming majority of books, schematics, etc. that I've seen use this scheme. \$\endgroup\$ Commented Feb 17, 2011 at 17:30
  • \$\begingroup\$ Wow, a downvote 3 years later... \$\endgroup\$ Commented Mar 29, 2014 at 2:42
  • 1
    \$\begingroup\$ ... and a comment 4 years after that! Well, canonical has a variety of definitions including "authorized; recognized; accepted" so I would argue that any abbreviations in common industry use could certainly be considered canonical, even if there is no prescribed standard. \$\endgroup\$ Commented Mar 6, 2018 at 14:06

The standard list of class designation letters is contained in IEEE 315, Clause 22.4. P is not a male connector, it is the most movable of a pair. And J is not a female connector, it is the most fixed of a pair. A connecting pair of sexless connectors, such as an APC-7, would follow this convention.

  • \$\begingroup\$ Say I had a pin header as a board-to-board connector. Which (P or J) should I use, as both boards are equally "movable"? KiCad seems to use "J" while EasyEDA uses "P". \$\endgroup\$ Commented Jan 19, 2020 at 6:07

A pin header with pins that slide into connectors with sockets would use class letter CP that stands for "connector adapter". That is if you have two PCBAs with female sockets on each board and they are ref des A1J1 and A2J1, and you have a header with male pins sticking out the top and bottom then the header would use class letter CP. Depending how you associate the coupler on your schematic diagrams would determine which parts list the coupler (connector) goes on.

If you have a pin (male) header on one board and a socket (female) header on a second board, where the two boards connect directly together, then arbitrarily call one board most movable and the other most fixed. (I would call the pin header by class letter P and the socket header by class letter J. How you show the assembly being put together on the assembly drawing might give you a clue as to which board is most movable and which is most fixed.

If you have two RF cables that connect together but each has an N-male connector, then a double female N connector, to connect the cables together, would use class letter CP (connector adapter) --Regards, Larry

  • 1
    \$\begingroup\$ Can you clarify what industry standard you are using to chose these reference designations? \$\endgroup\$ Commented Aug 16, 2021 at 22:23
  • \$\begingroup\$ The Class Designation Letters come from IEEE 315, Clause 22.4 (standard list of class letters). The Unit Numbering Method of applying reference designations is in ANSI/ASME Y14.44 \$\endgroup\$ Commented Aug 18, 2021 at 3:02

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