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Why is it spelled...?
Why does sane have long [a], while sanity has short [a]?
One important shortening rule, the Third Syllable Rule, states that if primary stress falls on the
third or fourth syllable from the end of word, the vowel in that syllable will be short, even if it is
the first vowel in a VCV string. That is the case with sanity: The third syllable from the end is
stressed, so its vowel is short. This Third Syllable Rule is most reliable with words adopted
directly from Latin; it is less reliable with words that are formed in English, such as, say, fatefully,
in which the third syllable from the end contains a long [a] (AES, 7.1-7.5).
But the suffix -ity, as in sanity, is involved in a more powerful case of the Third Syllable
Rule— the Suffix -ity Rule, which states that the third or fourth syllable back in a word that
ends -ity will always be short and will always be stressed. Thus, when -ity is added to words that
end in an unstressed syllable, the stress will shift to that syllable: civil
[|siv·l] vs. civility
[si·|vil·i·te]. Since -ity is such a productive suffix, the Suffix -ity Rule is very powerful
and useful (AES, 5.1--5.2.3).