The new compound Y(21)I(18)C(14)B(7) was prepared from YI(3), 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 P&onemacr;, a = 10.660(2) Å, b = 15.546(3) Å, c = 18.416(3) Å, 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.