The ancient Mesopotamian tablet showing a sophisticated right-triangle calculation is Plimpton 322.
Plimpton 322 is a clay tablet from the Old Babylonian period, generally dated to around 1800 BCE. It contains a table of numbers written in the sexagesimal, or base-60, system used by Babylonian scribes. Modern mathematicians recognize relationships among the numbers as examples of Pythagorean triples.
The tablet is named after George Arthur Plimpton, who owned it before it entered the collections of Columbia University. Its original purpose is debated. It may have been a mathematical exercise, a reference table, or part of a larger tablet that was later broken.
The tablet does not contain a statement of the Pythagorean theorem in the later Greek form. Its importance comes from demonstrating that Babylonian scholars could generate and organize complex numerical relationships centuries before Pythagoras. Interpretations of how its columns were produced remain an active scholarly topic.