The "lattice characters" have been introduced by Niggli, but so far there does not exist an exact general definition. Niggli's own definition defines only 40 characters out of the 44 now in use. The additional 4 characters are shown to require a further criterion, viz. the type of the reduced cell. © 1988.
de Wolff, P. M. (1988). Definition of Niggli’s lattice characters. Computers and Mathematics with Applications, 16(5–8), 487–492. https://doi.org/10.1016/0898-1221(88)90239-8