In early Old English, OE /a/ when not followed by a nasal was raised to OE /æ/ in certain phonetic environments.
OE /a/ becomes OE /æ/:
1. in monosyllables, except when followed by /w/, /h/, /r/ + consonant, or when preceded by /g/, /k/, or /s</; and
2. in polysyllables when /e/ or /i/ is in the following syllable.
OE /a/ remains /a/:
1. when followed by /w/; and
2. in polysyllables when /e/ does not stand in the following syllable.