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A number of polytypes have been found in (Cr, Fe)7C3 carbides. The basic structure of these carbides and polytypes can be considered as a stacking of layers parallel to the {110} planes in the basic cell. The symmetry of all possible polytypes of M7C3 carbides can be described by four space groups: Pb, P21/b, Pca21 and Pbca (Pca21 in two settings). A procedure for structure determination based on the analysis of electron diffraction patterns is proposed and applied to the polytypes: 2O, 3M, 4O, 4M, 5M1, 5M2, 5M3, 6O, 6M1, 6M2 and 8O (in Ramsdell notation).