Great English Vowel Shift
The Great English Vowel Shift is a keynote stage in the development of English. It is an enormous alteration in the pronunciation of English, and is one of the causes of modern irregularities in spelling vowels and particularly diphthongs. It also marks a difference between English and other European languages.
It may conventionally be dated as having occurred around the fifteenth century (1400 - 1500). It is one of the markers of the beginning of the Early Modern English phase of the development of our language.
Before the shift, in the Middle English period and before, the vowel letters in English represented sounds in more or or less the same way as they do in French, German and other current west European languages. That is to say, the broad sounds represented by the letters whose names are now pronounced eɪ, iː, aɪ, əʊ and [j]uː were realized as 'ah', 'eh', 'ee', 'oh' and 'oo' (IPA: /ɑː, e, iː, o and uː (or ɤ)/. (Alert readers may spot the correspondence of the IPA symbols with the values of these letters: they are the common values in the Roman alphabet as applied from the days of Latin.) (To see some of the other current sound values of these vowels, see Long vowel - short vowel, and the links therefrom.)
After the shift, the old pattern changed systematically (through the 'raising' of vowels) to its present form. Here A, E, I, O and U become 'eh', 'ee', 'eye', 'oh' and 'you' (IPA: /eɪ, iː, aɪ, əʊ and [j]uə/. Note that the sounds formerly represented by E and I have moved, in step, to be represented by A and E - at least in the letter names. This is one of the indications of the systematic nature of the change. Note also that only one letter is represented by a 'pure' vowel in IPA: four of them are diphthongized. These two factors are at the base of many of the awkwardnesses of English spelling of vowels.