Let denote a bipartite distance-regular graph with diameter D ≥ 4, valency k ≥ 3, and distinct eigenvalues θ0 > θ1 > · · · > θD . Let M denote the Bose-Mesner algebra of . For 0 ≤ i ≤ D, let Ei denote the primitive idempotent of M associated with θi . We refer to E0 and ED as the trivial idempotents of M . Let E, F denote primitive idempotents of M . We say the pair E, F is taut whenever (i) E,...