We extend the notion of a double normal of a convex body from
smooth strictly convex Minkowski spaces to arbitrary real, normed, linear spaces in two different ways. Then for both of the ways we obtain the following characterization theorem: a convex body $K$ in a Minkowski plane is of constant Minkowskian width iff every chord $I$ of $K$ splits it into two compact convex sets $K_1$ and $K_2,$ such that $I$ is a Minkowskian double normal of $K_1$ or $K_2.$ Furthermore, this theorem applied to the Euclidean plane is extended to $d$-dimensional Euclidean spaces.
Keywords:
body of constant (Minkowskian) width, Minkowski space, double normal, section, hyperplane