The notes of the Tristan chord are not unusual; they could be re-spelled to form a conventional half-diminished seventh chord. What distinguishes the chord is its unusual relationship to the implied key of its surroundings. When Tristan und Isolde was first heard in 1865, the chord was considered innovative, disorienting, and daring. Musicians of the twentieth century often identify the chord as a starting point for the modernist disintegration of tonality.
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This motif also appears in measures 6, 10, and 12, several times later in the work and at the end of the last act.
Much has been written about the Tristan chord's possible harmonic functions or voice leading (melodic function), and the motif has been interpreted in various ways. For instance, Schering (1935) traces the development of the Tristan chord through ten intermediate steps, beginning with the Phrygian cadence (iv6-V).
Vogel points out the "chord" in earlier works by Guillaume de Machaut, Carlo Gesualdo, Bach, Mozart, Beethoven, or Louis Spohr, as in the following example from Beethoven's Piano Sonata No. 18, tempo allegro:
What makes the Tristan motif different from earlier appearances of the same notes, in the eyes of many analysts, is its duration. In the Beethoven example, the E♭ resolves to D in approximately a quarter of the time it takes the G♯ to "resolve" to the A in the Wagner. In Wagner, the resolution is used merely in passing to a further chromatic dissonance (the A# in the following measure), rather than as a resting point in itself. In Beethoven, the notes' simultaneity may be considered to consist partly of nonchord tones; it is not a chord or harmonic entity in itself.
The Tristan chord's significance is in its move away from traditional tonal harmony, and even towards atonality. With this chord, Wagner actually provoked the sound or structure of musical harmony to become more predominant than its function, a notion which was soon explored by Debussy and others. In the words of Robert Erickson, "The Tristan chord is, among other things, an identifiable sound, an entity beyond its functional qualities in a tonal organization."
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