The close and mid-height front vowels of English (vowels of i and e type) have undergone a variety of changes over time and often vary by dialect.
Middle English had a long close front vowel /iː/, and two long mid front vowels: the close-mid /eː/ and the open-mid /ɛː/. The three vowels generally correspond to the modern spellings , and respectively, but other spellings are also possible. The spellings that became established in Early Modern English are mostly still used today, but the qualities of the sounds have changed significantly.
The /iː/ and /eː/ generally corresponded to similar Old English vowels, and /ɛː/ came from Old English /æː/ or /æːɑ̯/. For other possible histories, see English historical vowel correspondences. In particular, the long vowels sometimes arose from short vowels by Middle English open syllable lengthening or other processes. For example, team comes from an originally-long Old English vowel, and eat comes from an originally-short vowel, which underwent lengthening. The distinction between both groups of words is still preserved in a few dialects, as is noted in the following section.
Middle English /ɛː/ was shortened in certain words. Both long and short forms of such words often existed alongside each other during Middle English. In Modern English, the short form has generally become standard, but the spelling reflects the formerly-longer pronunciation. The words that were affected include several ending in d, such as bread, head, spread, and various others, including breath, weather, and threat. For example, bread was /brɛːd/ in earlier Middle English but came to be shortened and rhyme with bed.
During the Great Vowel Shift, the normal outcome of /iː/ was a diphthong, which developed into Modern English /aɪ/, as in mine and find. Meanwhile, /eː/ became /iː/, as in feed, and /ɛː/ of words like meat became /eː/, which later merged with /iː/ in nearly all dialects, as is described in the following section.
The meet–meat merger or the fleece merger is the merger of the Early Modern English vowel /eː/ (as in meat) into the vowel /iː/ (as in meet).