The anisotropy in the thermal transport decreases for the heavier chalcogens, but all three Pnma systems retain distinct "easy" and "hard" axes. For the R3 ̅ m, the transport along the x and y directions corresponds to transport in the ab plane, and is 4.4, 2.4 and 1.6 × larger than the transport in the cross-plane z direction (c axis) in Bi2S3, Bi2Se3 and Bi2Te3 respectively. As for the Pnma systems, the anisotropy therefore decreases with increasing chalcogen mass, but even for Bi2Te3 the cross-plane transport is 40 % smaller than the in-plane transport.