J is, along with U and W, one of the youngest letters in the Latin alphabet in its current graphic form, though it began as a mere handwriting variant rather than a wholly new invention. Throughout antiquity and the Middle Ages, as covered elsewhere in this atlas, a single letter I served for both the vowel /i/ and the consonant /j/ sound; medieval scribes, however, often lengthened the final I in a Roman numeral or the first I in a word with a small downward tail or hook purely as a decorative or clarifying flourish, with no intention of creating a new letter.
Renaissance grammarians, most notably the Italian Gian Giorgio Trissino in 1524, proposed formalising that tailed variant as a distinct letter for the consonant sound, freeing plain I for the vowel alone, but the reform spread slowly and unevenly: French and Spanish printers adopted the distinction over the following two centuries, while English dictionaries continued to alphabetise I and J together as late as the early 19th century. Several Renaissance-era ciphers covered elsewhere in this atlas, including the Theban alphabet, were fixed in print before or during this transition and still use a single sign for both I and J. J's numeral use, when it appears at all, is only as a decorative final variant of Roman numeral I, never as an independently valued numeral.