LENGTHENING OF VOWELS IN OPEN SYLLABLES

[An "open" syllable is a syllable ending in a vowel (e.g., the first syllable of mi-ser). A "closed" syllable, on the other hand, is a syllable ending in one or more consonants (e.g., the first syllable of mis-ter).]

During the 13th century, the Middle English short vowels /a/, /e/, and /ô/ lengthened in open syllables.

/a/ > /a:/
/e/ > /e:/
/ô/ > /ô:/

Later in the 13th century, ME /I/ and /U/ sometimes lengthened in open syllables, but with simultaneous lowering of the vowel.

/I/
(sometimes) >
/e/
/U/
(sometimes) >
/o/

A final /I/ or /i/ coming from OE /ig/ (normally spelled -ig) usually prevents lengthening of the preceding vowel in an open syllable. So, for example, OE /a/ in manig "many" did not lengthen.

Lengthening in open syllables did not occur if the stressed syllable was followed by two unstressed syllables.