One chord in Wagner's Tristan has kept music theorists arguing ever since.
The opening of Tristan und Isolde contains four notes, F, B, D sharp and G sharp, that refuse to settle into a key. Theorists still disagree about what the chord is and what it does. Many see it as a first step away from traditional tonal harmony.
The Tristan chord is heard in the very first phrase of Richard Wagner's opera Tristan und Isolde, as part of the musical motif associated with Tristan, and it recurs throughout the work up to the end of the final act. On paper its notes are not exotic. Respelled, they form a half-diminished seventh, a chord found in plenty of earlier music, including works by Bach, Mozart, Beethoven and Chopin.
What makes it unusual is its context. The music around it implies the key of A minor, but the chord does not behave the way the harmony textbooks say it should. Is it a French augmented sixth chord with a note that leans onto the next one? A chord built on the second, fourth or seventh degree of the scale? A dominant chord in disguise, decorated with passing dissonances? Serious analysts have argued for each of these readings, and others, and there is still no consensus.
That ambiguity is the point. By making the colour of a chord matter more than its function, Wagner opened a door that Debussy and others walked through. Arnold Schoenberg's circle saw Tristan as a forerunner of atonality, and Alban Berg quoted the chord in his Lyric Suite as a homage. Not everyone accepts that lineage: the scholar Jacques Chailley called it preposterous, insisting the chord's tensions are rooted in ordinary tonal logic.
Its fame has invited jokes. Debussy set it to the words 'je suis triste' in Pelléas et Mélisande and parodied the opera's opening in 'Golliwogg's Cakewalk'. Benjamin Britten made it hiccup through the orchestra in Albert Herring when the hero drinks spiked lemonade. And Paul Lansky built an electronic piece on it in 1973 that Radiohead later sampled in 'Idioteque'.
Source: Wikipedia — Tristan chord · Text summarised from Wikipedia (CC BY-SA 4.0)
