Remark 2

If (X, f) is the Dynkin graph of a reduced root system R, the Dynkin graph of the inverse system can be identified with (X, f^{-1}). In other words, the diagram associated to the Dynkin graph of R^- can be obtained from that associated to the Dynkin graph of R by reversing the inequality signs. If R is irreducible, we see that R is isomorphic to R^- unless R is of type B_l or C_l, in which case R^- is of type C_l or B_l.