Found this answer but I'm still confused, too.

"This lettering ... was the result of developments in the English alphabet. The letter J is a late variant of the Latin I, which gradually differentiated from I in function as well as form until, in the seventeenth century, the distinction between J as a consonant and I as a vowel was fully established. Similarly, the letters V and U are varieties of the same character—the U being simply a cursive form of the letter V, with which it was formerly used interchangeably. U and V were not given separate alphabetical positions until about 1800. The letter W, or double U, sometimes seen as a double V is a survival of this use."