International
Tables for Crystallography Volume A Spacegroup symmetry Edited by Th. Hahn © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. A, ch. 1.3, p. 6
Section 1.3.2. Notes on symmetry elements and symmetry operations^{a}Institut für Kristallographie, RheinischWestfälische Technische Hochschule, Aachen, Germany 
For the e glide planes, new graphical symbols were introduced (cf. Sections 1.4.1 , 1.4.2 , 1.4.3 and Note iv in 1.4.4 ); they are applied to the diagrams of the relevant space groups: Seven orthorhombic A, C and Fspace groups, five tetragonal Ispace groups, and five cubic F and Ispace groups. The e glide plane occurs only in centred cells and is defined by one plane with two perpendicular glide vectors related by a centring translation; thus, in Cmma (67), two glide operations a and b through the plane xy0 occur, their glide vectors being related by the centring vector ; the symbol e removes the ambiguity between the symbols a and b.
For five space groups, the Hermann–Mauguin symbol has been modified:

The new symbol is now the standard one; it is indicated in the headline of these space groups, while the former symbol is given underneath.
For the k glide planes, no new graphical symbol and no modification of a spacegroup symbol are proposed.
References
Flack, H. D., Wondratschek, H., Hahn, Th. & Abrahams, S. C. (2000). Symmetry elements in space groups and point groups. Addenda to two IUCr Reports on the Nomenclature of Symmetry. Acta Cryst. A56, 96–98.Wolff, P. M. de, Billiet, Y., Donnay, J. D. H., Fischer, W., Galiulin, R. B., Glazer, A. M., Senechal, M., Shoemaker, D. P., Wondratschek, H., Hahn, Th., Wilson, A. J. C. & Abrahams, S. C. (1989). Definition of symmetry elements in space groups and point groups. Report of the International Union of Crystallography Adhoc Committee on the Nomenclature of Symmetry. Acta Cryst. A45, 494–499.
Wolff, P. M. de, Billiet, Y., Donnay, J. D. H., Fischer, W., Galiulin, R. B., Glazer, A. M., Hahn, Th., Senechal, M., Shoemaker, D. P., Wondratschek, H., Wilson, A. J. C. & Abrahams, S. C. (1992). Symbols for symmetry elements and symmetry operations. Final Report of the International Union of Crystallography Adhoc Committee on the Nomenclature of Symmetry. Acta Cryst. A48, 727–732.