# Is there a convention for the polarity of voltage specified as $V_{XY}$? [duplicate]

Inspired by this answer, which includes the image:

Which solicited the comment:

No, you have the polarity of Vce shown backwards in the diagram [...]

The diagram seems consistent to me: the arrows go from the thing in the 2nd letter of the subscript, to the 1st letter of the subscript, in each case.

We might see similarly subscripted voltages in contexts besides BJTs: we also see them in FETs, or things that might not be transistors. Thus, given an arbitrary $V_{XY}$ to measure, the procedure is:

1. place positive lead of voltmeter on X
2. place negative lead of voltmeter on Y

Is this always the convention? Is it a more explicit definition? Or does it change, based on the thing in question, or even which part of the thing being discussed?

• Olin's comment was wrong - because he didn't notice that a subscript pair had had their order changed. ie Vce = - Vec. He noted this himself further down the discussion. | So - yes there is a convention. Vxy means that you measure the voltage at x relative to a zero point or reference at Y. If X is more positive than Y then Vxy is positive. If x is negative relative to Y then the numeric value of Vxy will be negative. Jan 29 '14 at 13:52
• As @Russell already pointed out, my comment to that answer was wrong because other things were silently changed from the original question and I didn't notice those changes. Vxy is voltage at X relative to Y, and all three voltages in the diagram above are shown correctly. That points out another convention, which is when a arrow is shown, it represents the voltage of the point at the head of the arrow relative to the one at the tail. To give a concrete example, Vbe is usually around 700mV in normal operation of a NPN transistor (Veb would be -700mV). Jan 29 '14 at 14:08
• Regardless of the situation that led to this question, I think it's a good question to have here. Perhaps I should edit it to remove the anecdotal bits, and just ask the question. Jan 29 '14 at 19:11

I was always taught that $V_{XY}$ would be the voltage of X relative to Y, and $V_{YX}$ would be the Voltage of Y relative to X. So as I was taught, the answer is yes, this is always the convention.

Here is what Wikipedia has to say in "Double subscript notation":

In electronics, the notation is usually used to indicate the direction of current or voltage ...

and later:

$V_{CB}$ means the "V" from C to B. In this case, V would probably stand for voltage, C would probably 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".

They cite as a reference:

^ Moses E. Zarudi, Shenkman, Arieh L. Shenkman (1998), Circuit Analysis for Power Engineering Handbook, Springer, ISBN 0-412-83180-5

Another reference: "Basic Electrical Engineering" By Kulshreshtha, p398

In terms of measuring with a meter, it is nicely described in All About Circuits: Kirchhoff's Voltage Law (KVL)

I now see that this question was also addressed here: Double-Subscript Notation for Voltages

• It would be good if you could provide a good reference. Jan 29 '14 at 19:10
• @PhilFrost Yes I agree ... so far just Wikipedia. If I can come up with something better, I'll add it; but I may need to wait until I get off work :-)
– Tut
Jan 29 '14 at 19:15
• @PhilFrost I've added a few references.
– Tut
Jan 29 '14 at 19:47