Lagrange started the theory of universal quadratic forms in 1770 by
proving his celebrated Four Squares Theorem, which in current language is
expressed by saying that the form x2+y2+z2+t2 is universal. The eighteenth
century was closed by a considerably deeper statement { Legendre's Three
Squares Theorem of 1798; this found exactly which numbers needed all four
squares. In his Theorie des Nombres of 1830, Legendre also created a very
general theory of binary quadratics.