No musical phrase affects die-hard Wagnerites as deeply as the Tristan chord, “the password, the cipher for all modern music,” as the German conductor Christian Thielemann has called it. This revolutionary break with harmonic convention—a combination of F, B, D-sharp, and G-sharp played by cellos, clarinets, bassoons, oboes, and an English horn, followed by a sequence whose haunting lack of resolution points the way to the once unimaginable realm of atonalism—appears in the opening measures of Richard Wagner’s music drama Tristan und Isolde . It hovers in the imagination much as it does in the concert hall, with a primal insistence that never diminishes no matter how often one hears it. The Tristan chord resounded once again at the Metropolitan Opera on March 9, at the premiere of its tenth new mounting of the epic work it first staged in 1886. This was an event eagerly anticipated among opera fanatics for several reasons.…