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{{short description|Type of notation used in engineering documentation}}
{{use dmy dates|date=September 2023|cs1-dates=y}}
{{use list-defined references|date=September 2023}}
In [[engineering]], '''double-subscript notation''' is a notation used to indicate some variable ''between'' two points (each point being represented by one of the subscripts). In [[electronics]], the notation is usually used to indicate the direction of [[electrical current|current]] or [[voltage]], while in [[mechanical engineering]] it is sometimes used to describe the [[force]] or [[stress (physics)|stress]] between two points, and sometimes even a component that spans between two points (like a beam on a bridge or [[truss]]). Although there are many cases where multiple subscripts are used, they are not necessarily called ''double subscript notation'' specifically.<ref name="Shenkman-Zarudi_1998"/>

== Electronic usage ==
IEEE standard 255-1963, "Letter Symbols for Semiconductor Devices", defined eleven original quantity symbols expressed as abbreviations.<ref name="IEEE_1963"/>
{| class="wikitable"
!Symbol
!Quality
|-
|A
|[[Anode]]
|-
|AV
|Average
|-
|B
|[[Bipolar junction transistor|Base]]
|-
|C
|[[Bipolar junction transistor|Collector]]
|-
|E
|[[Bipolar junction transistor|Emitter]]
|-
|G
|Gate
|-
|J
|Semiconductor terminal
|-
|K
|[[Cathode]]
|-
|M
|Maximum
|-
|Min
|Minimum
|-
|X
|Node (of a circuit)
|}

This is the basis for a [[convention (norm)|convention]] to standardize the directions of double-subscript labels. The following uses [[transistor]]s as an example, but shows how the direction is read generally. The convention works like this:

<math>V_\mathrm{CB} \ </math> represents the [[voltage]] from C to B. In this case, C would denote the collector end of a transistor, and B would denote the base end of the same transistor. This is the same as saying "the voltage drop from C to B", though this applies the standard definitions of the letters C and B.<ref name="Graf_1973"/> This convention is consistent with IEC 60050-121.<ref name="IEC_60050-121"/>

<math>I_\mathrm{CE} \ </math> would in turn represent the current from C to E. In this case, C would again denote the collector end of a transistor, and E would denote the emitter end of the transistor. This is the same as saying "the current in the direction going from C to E".<ref name="Graf_1973"/>

[[Power supply pins on integrated circuits]] utilize the same letters for denoting what kind of voltage the pin would receive. For example, a power input labeled V<sub>CC</sub> would be a positive input that would presumably connect to the collector pin of a [[BJT]] transistor in the circuit, and likewise respectively with other subscripted letters. The format used is the same as for notations described above, though without the connotation of V<sub>CC</sub> meaning the voltage from a collector pin to collector pin; the repetition avoids confusion as such an expression would not exist.

The table above shows only the originally denoted letters; others have found their way into use over time, such as S and D for the Source and Drain of a [[field-effect transistor|FET]], respectively.

==References==
{{reflist|refs=
<ref name="Shenkman-Zarudi_1998">{{cite book |title=Circuit Analysis for Power Engineering Handbook |author-first1=Arieh L. |author-last1=Shenkman |author-first2=Moses E. |author-last2=Zarudi |date=1998 |edition=1 |publisher=[[Kluwer Academic Publishers]] |publication-place=Dordrecht, Netherlands |isbn=0-412-83180-5}}</ref>
<ref name="IEEE_1963">{{cite book |title=IEEE 255-1963 - Standard Letter Symbols for Semiconductor Devices |publisher=[[IEEE]] |date=1963}}</ref>
<ref name="Graf_1973">{{cite book |title=Modern Dictionary of Electronics |author-last=Graf |author-first=Rudolf F.| publisher=[[Howard W. Sams & Co., Inc.]] |date=1973 |edition=4 |isbn= |pages=683–684}}</ref>
<ref name="IEC_60050-121">{{cite web |title=IEC 60050-121 |publisher=[[International Electrotechnical Commission]] |url=https://www.electropedia.org/iev/iev.nsf/display?openform&ievref=121-11-27 |access-date=2023-09-30}}</ref>
}}

[[Category:Notation]]
[[Category:Electronic engineering]]

{{engineering-stub}}

Double subscript notation - Revision history

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