J \J\ (j[=a]).
   J is the tenth letter of the English alphabet. It is a later
   variant form of the Roman letter I, used to express a
   consonantal sound, that is, originally, the sound of English
   y in yet. The forms J and I have, until a recent time, been
   classed together, and they have been used interchangeably.
   [1913 Webster]

   Note: In medical prescriptions j is still used in place of i
         at the end of a number, as a Roman numeral; as, vj,
         xij. J is etymologically most closely related to i, y,
         g; as in jot, iota; jest, gesture; join, jugular, yoke.
         See I. J is a compound vocal consonant, nearly
         equivalent in sound to dzh. It is exactly the same as g
         in gem. See Guide to Pronunciation, [sect][sect] 179,
         211, 239.
         [1913 Webster]

J is a consonant in English, but some nations use it as a vowel --
than which nothing could be more absurd.  Its original form, which has
been but slightly modified, was that of the tail of a subdued dog, and
it was not a letter but a character, standing for a Latin verb,
_jacere_, "to throw," because when a stone is thrown at a dog the
dog's tail assumes that shape.  This is the origin of the letter, as
expounded by the renowned Dr. Jocolpus Bumer, of the University of
Belgrade, who established his conclusions on the subject in a work of
three quarto volumes and committed suicide on being reminded that the
j in the Roman alphabet had originally no curl.