Middle and Old English had extra letters we have now lost: the eth, wynn, yogh and thorn. They represented the th sound and the w. When printing came along, they used movable type that obviously lacked these letters, coming from the continent, so they substituted y for thorn and z for yogh. So this has lead to a lot of mispronounced names and creation of new words. The best examples are ye old shoppe, where the ye is really the; or Mackenzie or Menzies, which should really be said Mackengie and Mengies.
Ignorance