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in -> inner

out -> outer / (outter?)

What is the history or set of rules behind why 'inner' doubles the 'n' but 'outer' doesn't double the 't'?

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If the n were not doubled, you would get iner, as in liner, which would be intelligible; hence, the double n. – Jimi Oke Apr 2 '11 at 1:26
up vote 21 down vote accepted

Many English words have a double consonant when following a short vowel. Consider "biter" and "bitter": the double-t signals that the vowel should be the short i vowel. Thus, you write "inner".

For "outer", "out" is already a two-letter vowel, so it doesn't change sound when you add on other letters at the end, hence "outer".

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protected by tchrist Dec 4 '14 at 14:11

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