from a proper accounting of stretching free energy in curved geometries. Our approach is to choose a geometry as an ansatz and compute the corresponding interfacial and stretching free energies in a manner consistent with calculations for cylindrical, spherical, and lamellar phases. The structure, revealed by scattering and electron microscopy,[@Hajd+94; @Schu+96] studies, can be described as follows,[@nitsche; @anderson90].

Consider first the D geometry, which is easier to visualize. A skeleton formed of the bonds of a diamond lattice is shown in Figure \[fig:skeletons\]. Two such lattices interpenetrate, analogous to the interpenetration of two simple cubic lattices in a BCC structure. Now imagine swelling the bonds in these lattices into tubes of a finite diameter. The walls of these tubes are a rough approximation of the experimentally observed “dividing surface” separating the regions containing the two blocks. The volume contained within the tubes corresponds to the region inhabited by the low volume-fraction monomer.

To model the D geometry, we use a self-dual minimal surface, called the Schwartz D (diamond) surface, which partitions space into two identical interpenetrating regions, each of which contains and is topologically equivalent to a diamond bond-lattice [@thomas86; @anderson88; @anderson90]. Within each of these regions is a dividing 