The [1][2] element of the anisotropic atomic displacement matrix B, which appears in the structure-factor term as: T = exp{-1/4 sum_i[sum_j(B^ij h_i h_j a*_i a*_j)]} h = the Miller indices a* = the reciprocal space cell lengths These matrix elements may appear with atomic coordinates in the ATOM_SITE category, or they may appear in the separate ATOM_SITE_ANISOTROP category, but they may not appear in both places. Similarly, anisotropic displacements may appear as either B's or U's, but not as both. The unique elements of the real symmetric matrix are entered by row. The IUCr Commission on Nomenclature recommends against the use of B for reporting atomic displacement parameters. U, being directly proportional to B, is preferred.