Note: Do not rely on this information. It is very old.
YY, the twenty-fifth letter of the English alphabet, is sometimes a vowel, sometimes a consonant. It comes from the Greek, in which language it is equivalent to u. In Latin it is used in words derived from Greek, and probably had the same sound. This was also its sound in Anglo-Saxon. Thus lyyte, afterwards luit, became the lute of Norman French; and hus, pronounced hoos, was the Anglo-Saxon form of "house" (haus). It also represents the Saxon g. Thus gard becomes yard; geol, yule.