Let’s start with the potion, which we began exploring (in terms of the story, in any event) HERE. Musically, we always hear Wagner’s enigmatic “Tristan” chord when they’re referring to the potion. The famous opening of the opera becomes something like a “potion” motif.
The opera opens with this motif, played three times in a bizarre sequence which keeps opening up again just as it’s about to resolve:
We hear that motif in Act Two, as Tristan sings “O heil dem Tranke, Heil seinem Saft! Heil seines Zaubers hehrer Kraft!” (O hail to the drink, hail to its juice! Hail to the lofty strength of its magic!):
…and in Act Three, when Brangäne pathetically tells Isolde that “Des Trankes Geheimniß entdeckt’ ich dem König” (I revealed to the King the mystery of the potion):
I’ve always thought Wagner made a strange choice in that last moment, bringing on Brangäne to apologize for drugging the lovers, as it were, at the very end when Tristan and Kurwenal are both dead and Isolde is about to sing her Liebestod. Usually I find myself wanting to yell at Brangäne, “Are you still talking about that? That was Act One!” But I think that’s the point: the opera begins and ends with the potion, the potion is the alpha and the omega and the core of the story of Tristan und Isolde.