Beginning in the Old English period and continuing in Middle English, most short vowels in unstressed syllables lost quality, converging on the midcentral short vowel /'/. Thus, in unstressed syllables,
| OE /a/ |
>
|
ME /'/ |
| OE /e/ |
>
|
ME /'/ |
| OE /o/ |
>
|
ME /'/ |
| OE /u/ |
>
|
ME /'/. |
The short vowel OE /i/ in general remained in unstressed syllables as ME /I/.
In Present-Day English, virtually all unstressed vowels are either /'/ or /I/.