From David Crystal's "Encyclopedia of the English Language":
Page 260:
J
The history of this letter in English dates only from the medieval period. Originally a graphic variant of i (a lengthened form with a bottom-left facing curve), it gradually came to replace i whenever that letter represented a consonant, as in major and jewel. The lower-case distinction did not become standard until the mid-17th century, and there was uncertainty about the upper-case even as late as the early 19th century.
Page 263:
U
The ancestor of U is to be found in the Semitic alphabet, eventually emerging in Latin as V used for both consonant and vowel. The lower-case letter developed as a smaller and rounded form in uncial script. In Middle English both v and u appear variously as consonant and vowel, in some scribal practice v being found initially and u medially. This eventually led to v being reserved for the consonant and u for the vowel, though it was not until the late 17th century that this distinction became standard.
Bingley
