trigonal system

In crystallography, the rhombohedral (or trigonal) crystal system is one of the seven lattice point groups, named after the two-dimensional rhombus. A crystal system is described by three basis vectors. In the rhombohedral system, the crystal is described by vectors of equal length, of which all three are not mutually orthogonal. The rhombohedral system can be thought of as the cubic system stretched diagonal along a body. a=b=c; alpha=beta=gamma unequal to 90°. In some classification schemes, the rhombohedral system is grouped into a larger hexagonal system.
See more at Wikipedia.org...