The original Tristan chord is heard in the opening phrase of Richard Wagner's opera Tristan und Isolde as part of the leitmotif relating to Tristan. It is made up of the notes F, B, D, and G:
More generally, the term refers to any chord that consists of the same intervals: augmented fourth, augmented sixth, and augmented ninth above a bass note.
Background
The notes of the Tristan chord are not unusual; they could be respelled enharmonically to form a common half-diminished seventh chord. What distinguishes the Tristan chord is its unusual relationship to the implied key of its surroundings.
This motif also appears in measures 6, 10, and 12, several times later in the work, and at the end of the last act.
points out the "chord" in earlier works by Guillaume de Machaut, Carlo Gesualdo, J. S. Bach, Mozart, Beethoven, or Louis Spohr as in the following example from the first movement of Beethoven's Piano Sonata No. 18:
The chord is found in several works by Chopin, from as early as 1828, in the Sonata in C minor, Op. 4 and his Scherzo No. 1, composed in 1830. It is only in late works where tonal ambiguities similar to Wagner's arise, as in the Prelude in A minor, Op. 28, No. 2, and the posthumously published Mazurka in F minor, Op. 68, No. 4.
The Tristan chord's significance is in its move away from traditional tonal harmony, and even toward atonality. With this chord, Wagner actually provoked the sound or structure of musical harmony to become more predominant than its function, a notion that 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".
Analysis
Much has been written about the Tristan chord's possible harmonic functions or voice leading and the motif has been interpreted in various ways. Though enharmonically equivalent to the half-diminished seventh chord F7 (F–A–C–E), the Tristan chord can also be interpreted in many ways. Nattiez distinguishes between functional and nonfunctional analyses of the chord.
Functional analyses
Functional analyses have interpreted the chord in the key of A minor in many ways:
The chord is an augmented sixth chord, specifically a French sixth chord, F–B–D-A, with the note G heard as an appoggiatura resolving to A. (Theorists debate the root of French sixth chords.) The harmonic function as a predominant is intact, with the chord moving to V7. Note that, in this view, the G# resolving upward to A is the 2nd of 3 consecutive accented dissonances resolving by half step. Alfred Lorenz and others interpret it more broadly as an augmented sixth chord F–A–D, based after Hugo Riemann on the principle that there are only three chord functions: tonic, predominant, and dominant. The root is the second scale degree, B. The root is the fourth scale degree, D. Arend interprets it as "a modified minor seventh chord" F–B–D–G → F–C–E–A → F–B–D–A = D–F–A The chord is a secondary dominant, V/V, and thus also with a root on B. This favors the fifth motion from B to E, seeing the chord as a seventh chord with lowered fifth (B–D(D)–F–A).
Vincent d'Indy analyses the chord as a IV chord after Riemann's transcendent principle (as phrased by Serge Gut: "the most classic succession in the world: Tonic, Predominant, Dominant" ) and rejects the idea of an added "lowered seventh", eliminates "all artificial, dissonant notes, arising solely from the melodic motion of the voices, and therefore foreign to the chord," finding that the Tristan chord is "no more than a predominant in the key of A, collapsed in upon itself melodically, the harmonic progression represented thus:
, independently, sees the G as an appoggiatura to A, describing that
According to Jacques, discussing Dommel-Diény and Gut, "it is rooted in a simple dominant chord of A minor [E major], which includes two appoggiaturas resolved in the normal way". Thus, in this view it is not a chord but an anticipation of the dominant chord in measure three. Chailley did once write: