International
Tables for Crystallography Volume A Spacegroup symmetry Edited by Th. Hahn © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. A, ch. 4.2, p. 61

The c cell in the square system is defined as follows: with `centring points' at 0, 0; . It plays the same role as the threedimensional C cell in the tetragonal system (cf. Section 4.3.4 ).
Likewise, the triple cell h in the hexagonal system is defined as follows: with `centring points' at 0, 0; . It is the twodimensional analogue of the threedimensional H cell (cf. Chapter 1.2 and Section 4.3.5 ).