Why do we draw a line between Middle English and Early Modern English at ca 1500?
The main difference between middle and early modern English is pronunciuation of long stressed vowels. This change, known as the "great vowel shift" began in approximately 1500, although the process continued for a few centuries. The reasons behind the Great Vowel Shift are not fully understood, although some scholars have hypothesized that the mass migrations following the black plague may have led to mingling of regional pronunciations. One of the reasons the Great Vowel Shift is still significant for the modern student is that much of English spelling started to be standardized with the invention of print in the fifteenth century. Thus, some (but not all) of our vowels are written in ways reflecting earlier rather than current pronuciations.
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