J is the tenth letter in the English alphabet. It is assigned the number 10 in the conversion table where A=1, B=2, ..., Z=26. The letter J used to be used as the swash letter I, used for the letter I at the end of Roman numerals when following another I, as in XXIIJ or xxiij instead of XXIII or xxiii for the Roman numeral twenty-three. In some areas of physics, electrical engineering, and related fields, j is the symbol for the imaginary unit (the square root of −1).