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I remember seeing the notation "R1 // R2" in electronics books, meaning "(R1^-1 + R2^-1 )^-1", i.e. the resistance of R1 and R2 connected in parallel. What is the correct way of typesetting the "//" sign?

I don't think it's the same as \parallel, but I couldn't find any example of the sign.

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You want that the double bar behaves like a binary operation, as far as spacing is concerned. So

\newcommand{\parallelsum}{\mathbin{\|}}

if you like vertical bars, or

\newcommand{\parallelsum}{\mathbin{\!/\mkern-5mu/\!}}

if you prefer slanted bars.

enter image description here

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    How is \newcommand{\parallelsum}{\mathbin{\|}} different from \parallel? – endolith Nov 14 '12 at 14:40
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    @endolith \parallel is defined as a relation symbol; try typesetting $a\parallel b$ and $a\mathbin{\parallel}b$ to see the difference: the spaces around a relation symbol are wider than those around a binary operation. – egreg Nov 14 '12 at 15:35
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Wikipedia uses \| but doesn't distinguish it from \parallel. Personally, I'd use \| in text and \parallel in display math, since the latter has operator spacing.

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    \| and \parallel print the same symbol, but with different spacing: \| is ordinary, \parallel denotes a binary relation. – egreg Dec 11 '11 at 15:27
  • @egreg: It is a binary relation, right? So \parallel is more semantically correct? – endolith Nov 14 '12 at 14:37
  • @endolith: Oh wait. Maybe it's a binary infix operator, which should have closer spacing? R_1 \| R_2 = 5 vs testing that two lines are parallel: "AB \parallel CD is false"? – endolith Nov 14 '12 at 15:26
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IMHO \parallel perfectly suits the given situation. Alternatively, you may try \shortparallel from the amssymb package or \parallelslant from fourier. From my experience, when two circuit elements are connected in parallel, the formula for total value is written 'R = (R1^-1 + R2^-1 )^-1', as you put it, or 'R = R1 x R2' (using e.g. \times), the notation with \parallel is less popular with engineers.

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  • When two components are connected in parallel, the total value is R_1 \parallel R_2 = {R_1 R_2 \over R_1 + R_2}. Writing it using the bars notation can greatly simplify expressions, and keep the relationship clear as symbols are moved around. – endolith Nov 14 '12 at 14:44

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