Ancient Greek mathematicians first studied what we now call the golden ratio, because of its frequent appearance in geometry; the division of a line into "extreme and mean ratio" (the golden section) is important in the geometry of regular pentagrams and pentagons. According to one story, 5th-century BC mathematician Hippasus discovered that the golden ratio was neither a whole number nor a fraction (an irrational number), surprising Pythagoreans. Euclid's "Elements" () provides several propositions and their proofs employing the golden ratio, and contains its first known definition which proceeds as follows: