One large distortion (Au, twisting of the pentacene) can be observed in the top right corner as the principal distortion, followed by Ag, B1u (and B1u x Au = B1g) and B3g (and corresponding B2u). The Lshaped blocks appear because, in D2, B1u and B1g both transform as B1, retaining the C2(z) operation, and thus are degenerate. Similarly, in C2, B3g and B2u each transform as B, finally settling on C1.