I was reading an article that uses the phrase "Achilles' heel" to refer to Facebook's acquisition of Instagram, but it was written thus:

achilles heel

My main question is - was this a bad mistake or is making this kind of description in this way a common thing? If so, could you provide examples where others have taken common phrases and lowercased them.

A little more context in case it helps:

Because Facebook is essentially about photos, and Instagram had found and attacked Facebook’s achilles heel — mobile photo sharing.

  • Related english.stackexchange.com/questions/63199/…
    – user19148
    Commented Oct 11, 2012 at 18:03
  • 1
    @tchrist I'd put your comment as the answer on this one. There are examples of eponyms that have "gone native" as it were and lost their capitalization but I'm unable to find solid examples of this one.
    – Marcus_33
    Commented Oct 11, 2012 at 18:37
  • 4
    Is this just about the casing used, or is the lack of apostrophe also in question?
    – Alan Gee
    Commented Oct 11, 2012 at 18:44
  • @Alan: ...and even if it isn't, I'd like to read the answer to that.
    – SF.
    Commented Oct 11, 2012 at 19:27

2 Answers 2


Achilles is a proper noun. However, this ngram does seem to show some extremely slight/rare incidence of the lowercase form.

Google Ngram of Achilles heel

That doesn’t make it “right” or generally accepted.

  • While Jon Purdy's answer is sound and absolutely valid, I chose this as the correct answer simply because I've never seen Google Ngram Viewer. Thanks all! Now... about that darn apostrophe... :P Commented Oct 11, 2012 at 23:31
  • Can one also prompt Google ngrams to show the trends of apostrophe usage in the compound? Commented Jan 20, 2023 at 16:18

Much like a genericised trademark, many terms that were originally derived from proper names have evolved over time into ordinary words:

Therefore I would say it’s acceptable to use the lowercase form, achilles heel.

  • Yes, or nemetic < Nemesis, but all of those are adjectives derived from nouns, while Achilles tendon uses Achilles attributively, so it is still a noun. In similar constructs, the capital seems to stick around. For example, the Golgi apparatus is not yet ∗golgi, either, nor the Kuiper belt yet ∗kuiper. Perhaps someday.
    – tchrist
    Commented Oct 11, 2012 at 21:58
  • @tchrist: I have no problem generalising. The capital will depart anyway when the legend is forgotten.
    – Jon Purdy
    Commented Oct 11, 2012 at 23:08
  • @tchrist, the difference is that I'm not sure if I've heard of the "Kuiper belt", and I'm quite certain I've never encountered a "Golgi apparatus", whatever the heck that is; whereas Achilles' heel is a pretty common phrase. The closest parallel I can think of is Krebs cycle, but that seems to be called the citric acid cycle nowadays, probably because people want to give credit where it's due to Szent-Györgyi's contributions, but nobody can pronounce Szent-Györgyi (just ask my sister).
    – Marthaª
    Commented Oct 11, 2012 at 23:20

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