(i) The projection direction is parallel to a latticecentring vector. In this case, the projected plane cell is primitive for the centring types A, B, C, I and R. For F lattices, the multiplicity is reduced from 4 to 2 because ccentred plane cells result from projections along face diagonals of threedimensional F cells.
(ii) The projection direction is not parallel to a latticecentring vector (general projection direction). In this case, the plane cell has the same multiplicity as the threedimensional cell. Usually, however, this centred plane cell is unconventional and a transformation is required to obtain the conventional plane cell. This transformation has been carried out for the projection data in this volume.
Examples
(1) Projection along [] of a cubic Icentred cell leads to an unconventional quadratic ccentred plane cell. A simple cell transformation leads to the conventional quadratic p cell.
(2) Projection along [] of an orthorhombic Icentred cell leads to a rectangular ccentred plane cell, which is conventional.
(3) Projection along [] of an Rcentred cell (both in obverse and reverse setting) results in a triple hexagonal plane cell h (the twodimensional analogue of the H cell, cf. Chapter 1.2
). A simple cell transformation leads to the conventional hexagonal p cell.

