A following /r/ tends to lower vowels.
1. During late Middle English and throughout Early Modern English, /r/ following /e/ tended to lower ME /e/ to EME /a/. Generally, the process was reversed later, and /ar/ reverted to /er/, which subsequently became /'r/ (see 4).
2. Often, a following /r/ prevented the expected change of the Great Vowel Shift . Thus, ME /e:/, ME /o/, and ME /u/ might remain unshifted when followed by /r/.
3. The regular development of ME /a/ was to become EME /æ/. In the 17th century, however, EME /æ/ resulting from ME /a/ reverted to /a/ when it was followed by /r/.
4. After the phenomena described in 1 and 3 above, EME /I/, /e/, and /U/ lowered and centered to EME /'/ before /r/.