High Quality Content by WIKIPEDIA articles! The square root of 2, also known as Pythagoras' constant, is the positive real number that, when multiplied by itself, gives the number 2. er 2. Geometrically the square root of 2 is the length of a diagonal across a square with sides of one unit of length; this follows from the Pythagorean theorem. It was probably the first number known to be irrational. This ancient Indian approximation is the seventh in a sequence of increasingly accurate approximations based on the sequence of Pell numbers, that can be derived from the continued fraction expansion of sqrt{2}.