The first known use of the sign ∞, in John Wallis’s treatise De sectionibus conicis (1655). In the middle of the text: “esto enim ∞ nota numeri infiniti”, let ∞ be the sign of an infinite number. Two lines above, the height of each parallelogram is 1/∞ of the total height.John Wallis · Wikimedia Commons · public domain In 1655 the Englishman John Wallis, a professor at Oxford, divided a plane figure into infinitely many very thin parallelograms, each with a height equal to an infinitely small part of the total height. To write that part, he needed a sign for infinity, and invented one: