J, tenth letter of the English alphabet. One of the last letters added to the alphabet, j is a variant of the letter i, from which it became formally distinguished only with the advent of printing. Before that time, j was simply an elongated i, and the 2 characters were used interchangeably. Thus, in Roman numerals, j sometimes replaced i to signify 1. It has the sound of y in Latin, German, and Scandinavian languages. In Spanish it has the sound of h.