Few chords are as significant as the Tristan chord. The Tristan chord opens Richard Wagner's opera 'Tristan und Isolde' in a spectacular way. It is known for its depth and slight dissonance, which brings an uneasy feeling to the listener. This reaction on the part of the listener, of course, is intended. Throughout the opera, this chord, which is made up of the notes F, B, D#, and G#, does not resolve; rather, it seems to grow more and more unsettling as the drama continues, for it is symbolic of the tension and unfulfilled desire of Tristan. The beauty of the Tristan chord is that it resolves not to a tonic, but to another discord, and later to another. The Tristan chord only resolves at the end of the opera, following Isolde's Liebestod (Love-Death), which is a sort of dying rant. Interestingly, Wagner originally called the prelude to the opera the 'Liebestod', and the above-mentioned aria, 'Verklärung' (Transfiguration). It was Franz Liszt who assigned the famous name to the aria, when he called his piano transcription of the aria the 'Liebestod'.
The chord's development is a prime example of Wagner's music telling the story of the opera just as perfectly, if not more so, than the libretto.
Director of the Royal Opera House Antonio Pappano has a nice video summarizing the significance of the Tristan chord.
The chord is known today as the 'Tristan' chord - meaning, the chord from the opera 'Tristan und Isolde' - but the chord is not unique to the opera at all. It is heard, albeit briefly, in Beethoven's Piano Sonata No. 18, as well as in works by Mozart, Chopin, Liszt, and others. This is not to say that Wagner stole this chord. Even if he had heard it before, which is likely, what is fascinating about Tristan und Isolde is Wagner's use of it; the development of the chord.
Tristan und Isolde - which was composed after Wagner completed the second part of the Ring tetralogy, Die Walküre - is known for its emotional power. A tenor died, suddenly, shortly after singing the role of Tristan during Wagner's lifetime; two conductors have died while conducting the second act of the opera. The opera brings tears to its audience, and continues to fascinate scholars and even casual opera-goers today. This is for a number of reasons, but it is primarily the music that carries the way - and the music is led by the Tristan chord.