We have the statement of Carl Philipp Emmanuel Bach, confirmed by Forkel, Bach's earliest biographer, that his father composed five Cantatas for every Sunday and Festival of the ecclesiastical year. Concerted music was sung at Leipzig annually on forty-three Sundays and sixteen week-days. Bach therefore must have written at least 295 Cantatas. Of this number he composed at least thirty before 1723. Hence approximately 265 were written at Leipzig. But Bach's fertility does not appear to have outlived the year 1744. We have reason, therefore, to conclude that the 265 Leipzig Cantatas were written in the course of twenty-one years, that is, between 1723 and 1744. To complete that number Bach must have composed a new Cantata every month, a surprising but demonstrable conclusion.