What is so strange about the first chord of Wagner's Tristan?

The first chord of Wagner's Tristan und Isolde is strange because it does not clearly belong anywhere, and because the music then refuses to settle for most of an evening. The Tristan chord is made up of the notes F, B, D-sharp and G-sharp. Its notes are not unusual in themselves, and could be respelled as a common half-diminished seventh chord. What distinguishes it is its unusual relationship to the key of the music around it.

A common account of its importance is that it moves away from traditional tonal harmony, even toward atonality, making the sound of the harmony more prominent than its function. Wagner did not invent the notes. Martin Vogel finds the same chord in earlier works by Machaut, Gesualdo, Bach, Mozart, Beethoven and Spohr. What the opera adds is delay: Wagner was one of the first composers to use harmonic suspension over a whole work, and the cadences introduced in the prelude are not resolved until the finale of act 3. Full resolution comes only at the very end, after Isolde's "Liebestod", or Love-Death.

Does the chord really begin modern music? Experts disagree. Much has been written about its possible harmonic functions, and one analysis hears it as a French sixth chord with a G-sharp that resolves to A. Schoenberg called it a "wandering chord" that can come from anywhere. The theorist Jacques Chailley, by contrast, heard its chromaticism, built on appoggiaturas and passing notes, as a peak of tension within tonal music, and called the idea that Tristan was a prototype of atonality "preposterous".

The opera was inspired partly by Schopenhauer's philosophy and by Wagner's relationship with his muse Mathilde Wesendonck.