I came across this term for the first time less than a day ago in Rindler's book Relativity (2 Ed. paperback p40). It's the assertion "that 'between' any two inertial frames S and S' there exists an inertial frame S'' relative to which S and S' have equal and opposite velocities." I am not able to understand his two-line "proof." I tried searching Google and Wikipedia for more on this but nothing turned up (:surprised). I was wondering if someone here knows about this lemma and could explain it to me, or provide a link to some site that explains it. Thanks.