Descartes at the beginning of his work on Geometry (1637), which would now be called a treatise on Algebraical Geometry, in order to shew the relation between Algebra and Geometry considers four types of quadratic equations. It will be noticed that these are special forms, the constant term of a quadratic equation being as a rule, not a square, but the product of two lengths, and necessitating the use of Euc. VI, 13 to find the equivalent square.