The role of symmetry in crystal structure formation is revealed for the pol
ymorphous modifications of Tm2S3 and related compounds Tm15S22, Tm5S7, and
Tm8S11 using the procedure of selecting and analyzing close-packed planes w
ith cationic, anionic, and combined motifs. The laws of mutual agreement be
tween the cation and anion matrices determining the size of the unit cells
are examined and relationships between the specific cation volumes and symm
etry are traced.