The Great Vowel Shift was a massive sound change affecting the long vowels of English during the fifteenth to eighteenth centuries. Basically, the long vowels shifted upwards; that is, a vowel that used to be pronounced in one place in the mouth would be pronounced in a different place, higher up in the mouth. The Great Vowel Shift has had long-term implications for, among other things, orthography, the teaching of reading, and the understanding of any English-language text written before or during the Shift.