  Let G and G' be absolutely almost simple algebraic groups of types B and C
respectively, of rank at least 3, and defined over a number field K. We
determine when G and G' have the same isomorphism or isogeny classes of maximal
K-tori. This leads to the necessary and sufficient conditions for two
Zariski-dense S-arithmetic subgroups of G and G' to be weakly commensurable.
