1. Old English and Middle English
[An "open" syllable is a syllable ending in a vowel (e.g., the first syllable of mi-ser), and a "closed" syllable is a syllable ending in one or more consonants (e.g., the first syllable of mis-ter).]
Starting as early as the 10th century, and becoming noticeable in the written records from the beginning of the 13th century, long stressed vowels shortened in stressed closed syllables before certain consonant groups. The consonant groups in question are the consonant groups other than the consonant groups that caused lengthening (see "Lengthening of Vowels before Consonants" ): that is, consonant groups other than /mb/, /nd/, /ld/, /rd/, /rs/, /rq/, or /rl/. This shortening occurred whether or not the consonant group was part of the root of the word: a consonant group created by the addition of an inflectional ending, for example, also caused shortening.
Shortening did not always occur before /st/.
During the same period, shortening occurred in stressed syllables followed by two or more unstressed syllables, whether or not the stressed syllable was closed.
During the Middle English period, long vowels in unstressed syllables shortened. This rule applies also to words of French origin whose stress had been displaced to the first syllable.
2. Early Modern English
ME /o/ normally became EME /u/, by the Great Vowel Shift . In some words this vowel subsequently shortened to /U/. Examples are foot, good, wood, etc. (In some words this shortened vowel later unrounded to /'/: e.g., blood, flood--see "Unrounding of Vowels" .)