Should the noun position be singular or plural in the following sentence?
You should focus on the position(s) of your lips and tongue pronouncing this sound.
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This is a counter-intuitive, but although you are talking about two [or even three!] things (lips and tongue), you could well choose to use the singular because position means "the placement [of what is being considered] relative to other things", and so can be used to describe the situation as a whole: each thing/assemblage can only have one position at a time. This applies even if you're talking about a set-up involving lots of different things, eg
"The position/s of the chess pieces on the board".
You would be forced to use the plural in cases where the thing or set-up you are discussing changes its position over time and you are bringing this point out, for example:
"Throughout the pronunciation exercise you will move your lips and tongue into lots of different positions, some of which might feel unusual."