International
Tables for Crystallography Volume E Subperiodic groups Edited by V. Kopský and D. B. Litvin © International Union of Crystallography 2010 
International Tables for Crystallography (2010). Vol. E, ch. 1.2, p. 17
Section 1.2.13. Reflection conditions^{a}Freelance research scientist, Bajkalská 1170/28, 100 00 Prague 10, Czech Republic, and ^{b}Department of Physics, The Eberly College of Science, Penn State – Berks Campus, The Pennsylvania State University, PO Box 7009, Reading, PA 19610–6009, 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 (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.