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Why does eighteen not have two tees like roommate has two ems?

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observation appreciated! and respected! – Thale Mar 9 '12 at 15:39
Eighteen is not a compound word, and spellings were not standardized until the 19th century or so— after both eighteen and roommate were in use. – choster Mar 9 '12 at 15:58
up vote 7 down vote accepted

Eighteen is not formed out of 'eight' and 'teen'; it's a number 'eighteen' in its own right. Etymologically, it once was a compound, though, of 'eight' and 'ten'. Old English did have both 't''s: eahtatene, but the two t's had merged by Middle English.

Roommate, on the other hand, is a word from Modern English and is indeed a compound word 'room' + 'mate'.

So the real answer to your question "why are they different?" is "because eighteen is a much older word".

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