  We introduce circulant matrices that capture the structure of a
skew-polynomial ring F[x;\theta] modulo the left ideal generated by a
polynomial of the type x^n-a. This allows us to develop an approach to
skew-constacyclic codes based on such circulants. Properties of these
circulants are derived, and in particular it is shown that the transpose of a
certain circulant is a circulant again. This recovers the well-known result
that the dual of a skew-constacyclic code is a constacyclic code again. Special
attention is paid to the case where x^n-a is two-sided.
