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In some science and engineering literature, it is common to right the ordinal form of an unknown number m as mth. While the ordinal form of number 1 is first, it is also commonly written as 1st. I was wondering what would be the most appropriate way to write the ordinal form of (m-1)? (m-1)th or (m-1)st?

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    Could "second-to-last" work? Even if you don't know the exact number of items, it unambiguously means "the one before the last", so if "last" were the Nth item, "second-to-last" would be the (N-1)th. – Friendly Neighborhood Demon Mar 4 at 22:59
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(m-1)th is still appropriate. The expression in parentheses will still be read without the ordinal, so consider how it sounds when spoken—em-minus-oneth makes more sense than em-minus-onest.

I should also note that it's common to avoid the problem entirely through rephrasing (e.g. 'the value at index (m-1)'), but that may not be applicable to your field.

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    I was reading it as em-minus-first. – Blade Mar 3 at 23:40
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    @Blade While it may vary from person to person (software dev myself), em-minus-first sounds to me like the mth value minus the first value, rather than the value at m-1. – AmaiKotori Mar 3 at 23:50
  • Yeah, I would probably say em-minus-first, with the stress somehow avoiding any possible confusion, in which case I would write (m-1)st, if I had to - although I don't think I've ever actually written that, because as @AmaiKotori says, it's often easier to rephrase. But I would have no problem with someone saying em-minus-oneth and writing (m-1)th - I think it's just personal preference :) – DM_with_secrets May 2 at 8:33

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