FamousTristan chordTristan und Isolde (Prelude to Act I), 1865 · Richard Wagner
Prelude to Act I, bar 2: the first chord, on the downbeat after the cellos' soft opening phrase (the rising sixth A3–F4 and the fall to E4). Played by the woodwinds (oboes, English horn, bassoons), with the oboe's G♯4 on top rising chromatically G♯–A–A♯–B into bar 3, where the music reaches a dominant seventh on E.
Four notes, F, B, D♯ and G♯, heard in the second bar of the Prelude to Wagner's Tristan und Isolde, after the cellos' quiet opening phrase. Written as notes it looks like several different chords at once, and it does not settle where the ear expects. It became a symbol of longing, and of the strain on keys that later composers pushed further.
Also called Tristan, Tristan-Akkord
Sources: John Rothgeb, 'The Tristan Chord: Identity and Origin', Music Theory Online 1.1 (1995) · William Rothstein, 'The Tristan Chord in Historical Context: A Response to John Rothgeb', Music Theory Online 1.3 (1995) · Tristan chord (Wikipedia) · John Mangum, program note on the Prelude and Liebestod (Los Angeles Philharmonic / Hollywood Bowl)