  A $\mathbb{Z}^d$-graded differential $R$-module is a $\mathbb{Z}^d$-graded
$R$-module $D$ equipped with an endomorphism, $\delta$, that squares to zero.
For $R=k[x_1,\ldots,x_d]$, this paper establishes a lower bound on the rank of
such a differential module when the underlying R-module is free. We define the
Betti number of a differential module and use it to show that when the homology
$H(D)=\mathrm{ker}(\delta)/\mathrm{im}(\delta)$ of $D$ is non-zero and finite
dimensional over $k$ then there is an inequality $\mathrm{rank}_R D \geqslant
2^d$.
