What is the great vowel shift and how has it changed the phonetic make up of the English Language?

The great vowel shift, GVS, is a change in the vowel inventory of English. Before the GVS, the English vowel system was similar to that of other West Germanic Languages. The shift lead to the raising of the long vowels, which all moved up a spot in the vowel diagram. As the highest vowels, the 'i' became dipthongised to 'ai' and the 'u' became 'au'.