Note: Do not rely on this information. It is very old.
UU, the 21st letter of our alphabet, is said, together with v, w, y, to come from the ancient letter Vau (F). U and V in later times became interchangable, but V was sometimes retained as an initial letter, and U as a medial. The Anglo-Saxon u sound is still partially preserved in such words as butcher, full, etc., and particularly in the North Country, as also in German and some of the Romance languages, and in many English words now spelt with oo. The French u and German ü are alike in sound, and the same sound of the letter is to be found in Welsh and in the West Country.