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Studying possessive apostrophes I have searched on ELU the questions correlated to this matter.

I have found the following: «What is the correct possessive for nouns ending in s?» and «When did it become correct to add an 's' to a singular possessive already ending in 's'?»

Reading both the first one and the second one I am not able to understand* why "Achilles heel"† is correct, as we can read on Oxford Advanced Learner's Dictionary.

Precisely I am asking why we should write "Achilles heel" rather than "Achilles' heel".

Does it depend on etymological reasons strictly correlated to the Greek origin? if so, in different no mythological cases, should we write, for example, either "Achilles tendon" or "Achilles' tendon"?

* I am not able to understand the issue because there is no answer to my question.

Named after the Greek hero Achilles. When he was a little child, his mother held him below the surface of the river Styx to protect him against any injury. She held him by his heel, which therefore was not touched by the water. Achilles died after being wounded by an arrow in the heel.

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And should we apostrophate "Adams apple", "widows peak", etc.? – GEdgar Apr 4 '12 at 13:46
up vote 10 down vote accepted

We don't use an apostrophe because the tendon and heel in question do not belong to Achilles the slayer of Hector in the Iliad, but to someone else. The name "Achilles" is a noun functioning as an adjective in that sentence. Compare brick oven, Trek bicycle, macadam surface, etc.

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In the original ancient Greek expression the name Achilles isn't a proper noun but an adjective, /Ἀχίλλειος/. Grammatically speaking it isn't a genitive but it functions like one syntactically. It wouldn't be weird if an apostrophe were present to denote that, as the heel does belong to Achilles. However, I agree that it isn't necessary because of the reason you write. +1 – Irene Apr 4 '12 at 10:34
@Robusto - I see your "Achilles heel" and raise you "Adam's apple". – Malvolio Apr 5 '12 at 1:05
@Malvolio: You can't use that as your Judas goat. – Robusto Apr 5 '12 at 1:20
@Robusto -- couldn't come up with a better eponym. I think I'm getting Alzheimer's. Wait, what? – Malvolio Apr 5 '12 at 23:57
@Robusto -- in "seriousness", I don't think your example is pertinent. A Judas goat doesn't resemble a goat that once belonged to Judas (the way an Achilles heel and an Adam's apple do); it resembles Judas himself. – Malvolio Apr 5 '12 at 23:59

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