Richard Wagner’s opera Tristan und Isolde set the course of 20th century classical music by keeping the audience in suspense for four hours with a single unresolved chord.
In classical music, harmonic suspension is a technique whereby tension is created and released. Essentially, a note from a particular chord is carried over into another chord. That note clashes with the new chord, creating dissonance and tension. And then that note drops, settles down into a resolving chord, and releases the tension.
Harmonic suspension has been a component of classical music for a long time. In the mid-19th century, though, Richard Wagner took it to another level, and in doing so effectively created 20th century classical music. And it all began with one chord.
The Tristan chord hits right at the beginning of his 1865 opera Tristan und Isolde. It’s a dissonant, unsettled chord, it makes you squirm and it makes you yearn. It’s a sad chord, a chord without a home. Like the love story of Tristan and Iseult, it is full of unreleased tension. Wagner takes that chord and dives into a building harmonic suspension. He ruthlessly refuses to let us rest – the strains of the chord echo through the opera, always building, always drawing closer and closer to a climax, to a musical resolution, to a release from the longing and anxiety of the Tristan chord. But! For almost the entire opera, four and a half hours, Wagner grants the listener no release.
Wagner teases us with that release. Halfway through the opera, the two main characters’ song pulls right up to the point of resolution, but they are interrupted and the suspension continues. The release, when it comes, hits right at the emotional and dramatic climax of the opera. It feels like the music has been breathing in for four hours, and finally, at last, it can breathe out. The effect is devastating.
It is not an understatement to say that the Tristan chord set the course of classical music from then on. For me personally, I appreciate its influence on the score of my favourite film, Alfred Hitchcock’s Vertigo. Like Tristan und Isolde, Vertigo’s score builds and circles around a long-suspended resolution. It mirrors the forbidden love (obsession?) at the heart of the film.
The following YouTube videos explore the chord and harmonic suspension in more depth. Take your pick from Stephen Fry:
Or Antonio Pappano of the Royal Opera:
Or Neil Brand on Vertigo:
The Generalist Academy 
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