The oldest competition for an optimal shape (area-maximizing) was won by the circle. But if the fixed perimeter is measured by the line integral of |dx| + |dy|, a square would win. Or if the boundary integral of max(|dx|, |dy|) is given, a diamond has maximum area. For any norm in R 2 , we show that when the integral of (dx, dy) around the boundary is prescribed, the area inside is maximized by a ball in the dual norm (rotated by π/2).This "isoperimetrix" was found by Busemann. For polyhedra it was described by Wulff in the theory of crystals. In our approach, the EulerLagrange equation for the support function of S has a particularly nice form. This has application to computing minimum cuts and maximum flows in a plane domain.