Primeness of no polynomial matrices is of fundamental importance in mu
ltidimensional systems theory. In this paper we define a quantity whic
h describes the ''amount of primeness'' of a matrix and identify it as
the concept of grade in commutative algebra. This enables us to produ
ce a theory which unifies many existing results, such as the Bezout id
entities and complementation laws, while placing them on a firm algebr
aic footing. We also present applications to autonomous systems, behav
ioural minimality of regular systems, and transfer matrix factorizatio
n.