International
Tables for Crystallography Volume E Subperiodic groups Edited by V. Kopský and D. B. Litvin © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. E, ch. 1.2, p. 16
Section 1.2.13. Reflection conditions^{a}Department of Physics, University of the South Pacific, Suva, Fiji, and Institute of Physics, The Academy of Sciences of the Czech Republic, Na Slovance 2, PO Box 24, 180 40 Prague 8, Czech Republic, and ^{b}Department of Physics, Penn State Berks Campus, The Pennsylvania State University, PO Box 7009, Reading, PA 196106009, USA 
The Reflection conditions are listed in the righthand column of each Wyckoff position. There are two types of reflection conditions:
The general reflection conditions are the result of three effects: centred lattices, glide planes and screw axes. For the nine layer groups with centred lattices, the corresponding general reflection condition is . The general reflection conditions due to glide planes and screw axes for the subperiodic groups are given in Table 1.2.13.1.

Example: The layer group p4bm (L56)
General position 8d: and due respectively to the glide planes b and a. The projections along [100] and [010] of any crystal structure with this layergroup symmetry have, respectively, periodicity b/2 and a/2.
Special positions 2a and 2b: . Any set of equivalent atoms in either of these positions displays additional ccentring.