[size=150]Plimpton 322 (P322) is one of the most sophisticated scientific artifacts of the ancient world, containing 15 rows of arithmetically complicated Pythagorean triples. But the purpose of this table has mostly eluded scholars, despite intense investigation. We argue that the numerical complexity of P322 proves that it is not a scribal school text, as many authors have claimed. Instead, P322 is a trigonometric table of a completely unfamiliar kind and was ahead of its time by thousands of years. To see how, we must adopt two ideas that are unique to the mathematical culture of the Old Babylonian (OB) period, between the 19th and 16th centuries B.C.E.First we abandon the notion of angle, and instead describe a right triangle in terms of the short side, long side and diagonal of a rectangle. [br][br]Second, we must adopt the OB number system and its emphasis on precision. The OB scribes used a richer sexagesimal (base 60) system which is more suitable for exact computation than our decimal system, and while they were not shy of approximation they had a preference for exact calculation.[/size]