The new compound Y21I18C14B7 was prepared from YI3, Y, C, and B at 1450 K. The average crystal structure was determined from single-crystal X-ray data. The structure consists of pseudoorthorhombic twisted metal atom double layers containing quasi-molecular CBC units. These are sandwiched by pseudohexagonal iodine layers. Owing to the symmetry and metric misfit the structure must be described with a large triclinic unit cell (space group , a = 10.660(2) Angstrom, b = 15.546(3) Angstrom, c = 18.416(3) Angstrom, alpha = 82.49(2)degrees, beta = 85.01(2)degrees, gamma = 82.92(2)degrees, Z = 2), which is probably only a good space-averaged approximation. Significant deviations from the averaged structure are indicated by unusual displacement parameters and the results of high-resolution electron microscopy.