The Tristan chord is a chord that contains an augmented fourth, augmented sixth and augmented ninth above the root. Although other composers used this particular chord, composer Richard Wagner most famously used it with the pitches F, B, D# and G# in the beginning bars of his composition, "Tristan und Isolde." The chord occurs makes up a portion of Tristan's theme or leitmotif and is considered one of the most famous chords in all of music. The pitches could be respelled to form a standard half diminished seventh chord, but the relationship between the chord and what surrounds it in "Tristan und Isolde" is unusual.
The Tristan chord is one of the most hotly debated chords in music theory because theorists do not agree on exactly how to analyze it. It has been analyzed in both functional and nonfunctional theory approaches. Within each of these approaches, different interpretations of the chord exist, none of which can be proven necessarily correct or incorrect.
The key to understanding the Tristan chord — and the heart of the analysis debate — is that some of the notes can be interpreted as appoggiaturas. An appoggiatura is defined as an embellishing note, or a note that comes before a pitch more essential to the melody. In other words, some of the notes of the chords can be left out of the analysis, which drastically alters how the chord might be working.
Although many interpretations of the Tristan chord exist, Wagner himself accepted an interpretation by Czech professor K. Mayrberger, who analyzed the chord on the second degree (II) and treated the G# as an appoggiatura. Mayrberger saw this chord as somewhat split. He felt that the F was associated with the key of A minor, while the D# was related to the key of E minor.
The duality of the Tristan chord seen by Mayrberger caused many theorists to view the chord as foreshadowing the abandonment of traditional harmony toward approaches such as polytonality. Polytonality means that the composer uses more than one key simultaneously. Musicians thus hailed the Tristan chord as the epitome of contemporary harmony, but in reality, this chord is not "new" and is present in much of tonal music, including that of Ludwig Von Beethoven, Johann Sebastian Bach and Wolfgang Amadeus Mozart. Modern theorists often see the chord as Wagner's contemporary adaptation of harmony as a result.
The Tristan chord is so famous that it has been parodied or borrowed many times by composers, although it appears in a handful of spellings. Some of these parodies or borrowings are intentional homages to Wagner, but others are not. This is an important note, because normally, it is melody that is borrowed. With the Tristan chord, it is the specific sound created by harmonic intervals that composers latch onto and purposely replicate within various genres.