Diamond, known mainly in the cubic form, has a less common hexagonal form called lonsdaleite. Such structures are also observed in binary SiC, and the different phases are called polytypes, which are distinguished by the stacking of layers, usually labeled A/B/C: AB in 2H (2 layers; hexagonal system), ABC in 3C (3 layers; cubic system), ABCB in 4H (4 layers; hexagonal system), etc. .