On circuit boards, I see that transistors are "Q", but why Q? Shouldn't "TR" be transformer, so transistor can be "T" because transistors seem to be more common then transformers.

  • \$\begingroup\$ And for that matter, why are ICs "U"? \$\endgroup\$
    – HikeOnPast
    Apr 25, 2013 at 2:34
  • 2
    \$\begingroup\$ @hikeOnPast -- u = Greek Mu; uIC as in "Micro integrated circuit" (from the 50's) \$\endgroup\$ Apr 25, 2013 at 2:36
  • \$\begingroup\$ Early in the transistor era, I occasionally saw schematics with transistors designated as TRnnn. \$\endgroup\$ Apr 25, 2013 at 12:28
  • \$\begingroup\$ @HikeOnPast ICs are often called ICnn. \$\endgroup\$
    – starblue
    Apr 25, 2013 at 20:02
  • \$\begingroup\$ In the 1975 schematics for a Siemens Oscillar M07107 oscilloscope they use TsNNN. \$\endgroup\$
    – starblue
    Oct 19, 2015 at 13:06

3 Answers 3


There are claims that, after initial chaos, eventually the letter Q was chosen because of the popular TO18/TO39 case styles.

general view of TO18 case

Just look at it from the bottom:

bottom view of TO18 case

And of course, 'T' had been used for transformers since decades earlier.

Now, finding trustworthy sources available on the web... well, it hasn't been a lucky few minutes for me, but I know I read this somewhere in print years ago.

  • 2
    \$\begingroup\$ Ouch, that 2nd image came out bigger than intended! \$\endgroup\$
    – DarenW
    Apr 25, 2013 at 6:17
  • 2
    \$\begingroup\$ Image Size: FTFY. \$\endgroup\$
    – zebonaut
    Apr 25, 2013 at 19:28

Here's the rumor I have read somewhere: When transistors were new, they were that strange part no one was really using yet. "T" was already taken (transformer), and the not-so-often used letter "Q" was (i) not used for anything else and (ii) seemed appropriate for a not-so-often used type of component.

I have stopped worrying about how to call parts. The letters are not used consistently in different parts of the world - "Q" for transistors, for instance, is an American tradition; Europeans often call transistors "T". Except for Germany, where discrete semiconductors (diodes, transistors) tend to be called "V" in some companies even though the standard DIN EN 60617 suggesting the letter "V" is not even valid any more, iirc. The current standard, DIN EN 61346, suggests the letter "K" for transistors, but I've met no one who is really hardcore enough to actually use it.

Oh, and I've just found out that DIN EN 61346 has been withdrawn and replaced by IEC 81346. The German wikipedia article has a complete list about all the suggested reference designators, and you can clearly see how the goal of finding one big classification, valid for everything and anything from board level design to industrial automation in large installations, has led to a big table that has FUBARed everything. Search for the term "transistor" in the wikipedia article, and you'll find that you're supposed to use "KF"!

  • \$\begingroup\$ There are enough tube ("valve") amplifiers still around, especially for guitar players, that Vnnn (for "valve") will be around for a long, long time yet. \$\endgroup\$ Apr 25, 2013 at 12:30


  • The short answer is "because [someone] said so!" (Shockley? Moore?) and then it was codified
  • Might refer to the Q-point ("quiescent" point) as load-line analysis (Q-point analysis, or bias analysis) came of age around the same time as the advent of the transistor
  • FYI: "T" is transformer according to ANSI/IEEE

(My) Summary of the Codification History:

1950's Military defines standards for drafting to facilitate what we now call "lifecycle management"

1970's IEEE/ANSI tries to get in on the action and set a wide reaching commercial standard

1990's IEEE tries to update it, but realizes its pointless (e.g. good enough)

From Wikipedia (emphasis mine):

IEEE 200-1975 "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.


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