International
Tables for Crystallography Volume A Spacegroup symmetry Edited by M. I. Aroyo © International Union of Crystallography 2015 
International Tables for Crystallography (2015). Vol. A, ch. 1.5, p. 82


Figure 1.5.1.9
Rhombohedral lattice with a triple hexagonal unit cell a, b, c in obverse setting (i.e. unit cell a_{1}, b_{1}, c in Fig. 1.5.1.6c) and the three centred monoclinic cells. (a) Ccentred cells with ; with ; and with . The unique monoclinic axes are and , respectively. The origin for all four cells is the same. (b) Acentred cells with ; with ; and with . The unique monoclinic axes are , and , respectively. The origin for all four cells is the same. The fractions indicate the height of the lattice points along the axis of projection. 