A pictorial device used to represent pyroxenes. Octahedral M1 chains may have tetrahedra with triangles oriented in the same (S) or opposite (O) direction as those of the facing octahedron face.

In addition, when viewing down the c axis, the octahedra may appear in one of two possible orientations

A pyroxene structure may then be depicted as a cluster of interlocking I-beams, with O or S denoted at top and bottom and or in the middle.

If the orientations of stacked I-beams are the same, the crystal in monoclinic. If they are opposite, the crystal is orthorhombic.