During many decades, it was a general belief that a flag system could always be represented locally by the von Weber model. However, this is not the case and, in 1978, Giaro, Ruiz and the author brought to evidence a non-homogeneous model ([9]). More recently, the same model was found independently by Mormul ([20]). The presence of such non-homogeneous models sheds some doubts on Cartan's criterion since statements such as in [4], p. 89, [11], p. 328, and [22], p. 227, become inaccurate. It is our purpose here to prove the Cartan criterion with the added hypothesis of homogeneity and show as well that this criterion can be applied directly to higher order ordinary differential equations without the necessity of reducing them to first order equations. Finally, we examine first and second order single equations, these involving flag systems of length two and three respectively.(*) Entrata in Redazione il 1 agosto 1998. Indirizzo dell'A.: Rua Campos Sales 225,