Christophorus Clavius (Latinization of his birth name) was a German Jesuit mathematician and astronomer. He was the first to use parentheses to isolate mathematical expressions. He was commissioned by Pope Gregory XIII to write the basis for a reformed calendar. The calendar was instituted by Pope Gregory XIII in 1582 and gradually adopted throughout the world; the calendar, which came to be known as the Gregorian calendar, is still used today. Christopher Clavius (1538-1612)