The History of the Letter Y

The original version of the word king was pronounced cuning, and it was spelled C-Y-N-I-N-G. That use of Y for the vowel sound, U, passed into the Irish script. And since Old English happened to have that same sound, it also passed into Old English. So by the end of the Old English period, the letter Y and the letter I were representing the same vowel sound in both languages.