I

from Fancyclopedia 2 ca. 1959 and Fancyclopedia 1 ca. 1944

A letter whose so-called "long" sound is actually a close diphthong, as the Greeks and Romans recognized in spelling it ai and ae respectively. The mistake occurred in English, according to de Camp, when the vowel shift made E represent the Latin I-sound, and drove the I up to where there was nothing to do but become a diphthong.