Early in Old English (before written records), certain vowels in certain phonetic environments were "broken" into diphthongs.

Certain front vowels followed by certain velar consonants developed a "glide," as the tongue moved from articulating the vowels at the front of the mouth to articulating the consonants at the back of the mouth. The result was the production of a diphthong in the place of the "pure" vowel.

OE vowels followed by the OE velar consonants /h/, /l/, /r/, and /w/ broke according to the following rules.

/æ/, /æ:/
  '/, /æ:'/
/i/, /i:/ >
/i'/, /i:'/
> /e'/, /e:'/