A new construction of the self-dual, doubly-even and extremal [48, 24, 12] binary linear block code is given. The construction is much like that of a cyclic code from a polynomial. A zero divisor in a group ring with an underlying dihedral group generates the code. A proof that the code is of minimum distance twelve, without need to resort to computation by computer, is outlined. We also prove the code is self-dual, doubly even and that the code is an ideal in the group ring. The underlying group ring structure is used, which offers a number of useful generator matrices for the code. Interestingly, the construction involves unipotent elements within the group ring, and these lead to the creation of weighing matrices.