A theorem that describes how physically meaningful equations that describe observable phenomena involving n variables can be presented as an equation of n−m dimensionless groups, in which m is the number of fundamental dimensions such as mass, M, length, L, and time, T. The theorem allows the dimensionless groups to be determined from the variables. The π theorem was first proved by French mathematician Joseph Bertrand (1822–1900) and is named after American physicist Edgar Buckingham (1867–1940). See rayleigh’s dimensional analysis method.