I am almost ashamed to put this question, as it verges on the silly, but it is a real question, and Physics Forums has come to my rescue so often.... I am translating a physics article from Russian, and at one place the author puts quantities (such as σ for standard deviation, etc.) with a double subscript: one indicating the particle involved, and the second the axis upon which the quantity is projected. I am not one to ask the author to change his style (let alone the content), but I do have to change the Russian letter he uses for "particle" to an English subscript. He uses the first letter of the Russian word for "particle", so that with a projection to the x-axis, this looks like σчx. If I used "p", this would look too much like momentum. I can't think of another natural single letter. Any suggestions? I could then use a syllable instead of a letter: if I use, say, "par", then I have to come up with a natural way to combine them (without second-level subscripts such as σparx or σxpar): parx is obviously no good, but I am not sure about something like par(x), par-x, par.x, or par/x. So, any suggestions? Thanks.