Phonological history of English vowels

In the history of English phonology, there have been many diachronic sound changes affecting vowels, especially involving phonemic splits and mergers.
Great Vowel Shift and trisyllabic laxing.
The Great Vowel Shift was a series of chain shifts that affected historical long vowels but left short vowels largely alone. It is one of the primary causes of the idiosyncrasies in English spelling.
The shortening of ante-penultimate syllables in Middle English created many long–short pairs. The result can be seen in such words as,
Tense–lax neutralization.
Tense–lax neutralization refers to a neutralization, in a particular phonological context in a particular language, of the normal distinction between tense and lax vowels.
In some varieties of English, this occurs in particular before and (in rhotic dialects) before coda (that is, followed by a consonant or at the end of a word); it also occurs, to a lesser extent, before tautosyllabic .
In the Pacific Northwest, especially in the Seattle area, some speakers have a merger of with before . For these speakers, words with like "beg", "egg", "Greg", "keg", "leg" and "peg" rhyme with words with like "Craig", "Hague", "plague" and "vague".
Some varieties (including most American English dialects) have significant vocalic neutralization before intervocalic , as well. See English-language vowel changes before historic /r/.
Monophthongs.
Schwa.
Schwa syncope is the deletion of schwa. English has the tendency to delete schwa when it appears in a mid-word syllable that comes after the stressed syllable. Kenstowicz (1994) states that "... American English schwa deletes in medial posttonic syllables ...", and gives as examples words such as "sep(a)rate" (as an adjective), "choc(o)late", "cam(e)ra" and "elab(o)rate" (as an adjective), where the schwa (represented by the letters in parentheses) has a tendency to be deleted.
Vowel changes before historic /r/.
Mergers before intervocalic /r/.
Mergers before intervocalic "r" are quite widespread in North American English.
Mergers before historic coda /r/.
Various mergers before historic coda r are very common in English dialects.