 Space groups and their descriptions in International Tables for Crystallography

VolumeA, chapter1.4, section1.4.4: Wyckoff-position descriptions and letter convention, beginning with a for maximal site-symmetry order and ending with the general position. Companion primary table onlinelibrary.wiley.com/iucr/itc/Ac/ch3o4v0001/ , Table3.4.3.3, gives P1 general position1a.