  It is known that the notion of graded differential algebra coincides with the
notion of monoid in the monoidal category of complexes. By using the monoidal
structure introduced by M. Kapranov for the category of $N$-complexes we define
the corresponding generalization of graded differential algebras as the monoids
of this category. It turns out that this generalization coincides with the
notion of graded $q$-differential algebra which has been previously introduced
and studied.
