Somos defines a rational triangle as a triangle such that all three sides measured relative to each other are rational. Koblitz (1993) defined a congruent number as
an integer that is equal to the area of a rational right
triangle. The discovery that a right triangle of unit leg length has an irrational
hypotenuse (having a length equal to a value now known as Pythagoras's
constant) showed that not all triangles are rational.