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, §§ 179, 211, 239.