When a diphthong becomes a single "pure" vowel, the phenomenon is called "smoothing."

1. The Old English diphthongs '/, /æ:'/ and /e'/, /e:'/ smoothed in Middle English to produce the following results.

OE /æ'/
ME /a/
OE /æ:'/
ME /e:/
OE /e'/
ME /e/
OE /e:'/
ME /e/


2. In general, the Middle English diphthongs smoothed in Early Modern English, producing the following results.

i. ME /æi/ > EME /e/.

ii. ME /au/ > EME /ô/ except before a labial consonant, in which case ME /au/ > EME /a/ or /æ/.

iii. ME /eu/ fell together with ME /iu/ and in the 16th century > EME /ju/.

iv. ME /Ui/ and /ôi/ fell together as EME /ôi/. In some dialects, however, the two ME diphthongs remained distinct into the PDE period: in these dialects, ME /Ui/ > EME /ôi/, and ME /ôi/ > EME /'i/.

v. ME /ôu/ > EME /o/.