The opening moments of the Prelude to Tristan und Isolde has had probably more commentary, discussion and analysis than almost any other music in the history of Western classical music. What is surprising is that so many commentaries limit themselves to the first phrase. The music in subsequent phrases is rarely discussed. The subsequent phrases also stretch the boundaries of conventional harmony both in ways both similar to and different from the opening phrase. Those other cases will be examined later. First the opening passage in short score and audio is below:
While it is remarkable, it is neither unique nor unprecedented. The same chord was in use long before Wagner employed it in Tristan und Isolde. What is unique is that the chord is used repeatedly and in contexts not previously used. Due to its frequency and placement at key moments in Tristan und Isolde, the chord becomes emblematic of the whole milieu of the opera.
The chord is the first harmony heard in the opera at the start of the second bar of the Prelude and marked in red in the example above. Today it is known as a half-diminished chord or as a minor seventh with a flattened fifth. It occurs naturally on the seventh degree of the major scale and the second degree of the minor scale as shown below:
Before examining the Tristan case further, the following examples demonstrate that the half-diminished chord was not uncommon in the music of composers prior to Tristan und Isolde. For this survey and for reasons of time and practicality, it was not realistic to survey a wide range of composers and works. A few composer and specific idioms have been chosen for the reasons given below.
The Bach Chorales were surveyed. The four-part harmony idiom is mostly chordal. The intended chord or harmony is clear and easy to ascertain, perhaps more so than would be the case in contrapuntal works, where suspensions and passing notes may lend some ambiguity to any analysis of the underlying harmony.
The Beethoven Piano Sonatas were surveyed. The piano sonatas span most of Beethoven's years of creative output. The idiom lends itself to identifying half-diminished chords in those sections where there is clear chordal writing. Examples were not included where the harmonic basis was in doubt or there were suspensions and independent writing that may have more than one interpretation of the underlying harmony.
The Chopin works surveyed were limited to the Preludes and Etudes. Finally, some Wagner works prior to Tristan und Isolde were surveyed. This was limited to Das Rheingold, Die Walküre and Wesendonck Lieder.
Bach often used the half-diminished chord on the second scale degree in a major key. The sixth degree of the scale, the fifth of the chord, is flattened to create a half-diminished chord. Also used is the more familiar seventh chord on the second degree in the minor key. Furthermore, the use of a seventh chord on the seventh degree of the major scale is not uncommon.
Beethoven frequently uses the half-diminished chord in transition passages. Like his use of the Augmented Sixth and Neapolitan Sixth, the half-diminished chord is often employed during modulation.
In the Chopin Preludes and Etudes, the half-diminished chord is used sparingly but certainly present. Its role is often to unsettle one key in preparation for modulation to another. It is frequently used to enable modulation to and from unrelated or distant keys.
Wagner employed the half-diminished chord extensively prior to Tristan und Isolde, particularly in Das Rheingold and Die Walküre. It is not surprising that the chord would also be found in the Wesendonck Lieder, since these songs were written immediately prior to commencing work on Tristan und Isolde and are something of a study for the larger work, including some common musical elements.
In the works outlined above, the half-diminished chord functions as a substitute or secondary dominant. That is the seventh chord on the supertonic resolves to the dominant or dominant seventh in most cases. The seventh chord on the leading note resolves to the tonic in most cases. This is the case in both major and minor keys.
The full list of examples of the occurrence of the half-diminished chord in the surveyed works can be found in the appendix on the next page. For this discussion, a few typical samples are shown below, to demonstrate:
This example from the end of Chorale No 6 demonstrates a frequent practice in Bach's work of flattening the fifth of the supertonic seventh chord in a major key to create the half-diminished chord, which then resolves to the dominant seventh and which in turn resolves to the tonic.
This Bach example from Chorale No 8 shows a passage in D major. (The chorale is in A major but this phrase modulates to D major). The cadence employs the leading note seventh chord resolving to tonic. Unusually for Bach, and indeed for most standard, classical use of the half-diminished chord, this is in root position. The chord is mostly used (by Bach and Beethoven) in first inversion, since the diminished fifth between the root and fifth degree was considered a dissonance. This example, in root position, is exceptional.
This example also demonstrates that the half-diminished chord was most often deployed in first inversion. Sonata No 14 in C sharp minor (“Moonlight”) shows the chord used at a typical cadence of supertonic seventh followed by dominant seventh and then tonic.
In this example from Sonata No 18 the half-diminished chord is used in a prolonged manner over several bars. Its resolution is then to the diminished seventh, further weakening the sense of key, before treating the diminished seventh as the leading note seventh in the home key.
This example from the Chopin Etudes Opus 10 shows the half diminished in use for the full bar with a suspended B flat on the first beat "resolving" to an A flat on the second beat. While the half diminished chord is clearly present on the first beat, the suspended note adds to its tension, and makes the resolution seem like a consonant chord rather than a dissonant one that needs resolution. The half diminished chord is in root position in this example. It coincidentally uses the same notes as the first occurence in Tristan und Isolde. The difference is that in the Chopin example it is a chord on the supertonic which resolves to a dominant ninth chord in first inversion.
In this example from Das Rheingold, the half-diminished chords are supertonic sevenths resolving to dominant sevenths first in E flat minor then the same progression in D flat minor. The resolution to the tonic does not occur.
In this example from Die Walküre, the half-diminished chord is on the supertonic G sharp in the key of F sharp minor. The supertonic seventh is followed by a flattened supertonic seventh which is in turn followed by the tonic F sharp minor, which is then altered to a dominant seventh of notionally B minor.
The full list of examples in the appendix shows how frequently Wagner used the half-diminished chord prior to Tristan und Isolde. Its use is most often as a supertonic seventh in first inversion in the minor key, which resolves to the dominant or dominant seventh. In many instances, the resolution to the tonic does not occur.
To come to Tristan und Isolde, the half-diminished chord is liberated from its use on the second or seventh degree of the scale. Nevertheless, its role as a secondary dominant, that is a chord that resolves to the dominant or dominant seventh is maintained in many cases.
The famous opening phrase varies from the classical or traditional use in that the half-diminished chord occurs on the sixth degree resolving to the dominant seventh of what is notionally A minor. This is not so different to the use of the Augmented sixth or Neapolitan sixth in classical harmony.
In the Prelude to Tristan und Isolde, the half-diminished chord occurs several times. The resolution varies and, in some cases, does not resolve at all. A full list of these occurrences is also in the appendix. As the work progresses, the half-diminished chord occurs without resolution.
The “game” of spot the half-diminished can be continued throughout the opera. In summary, the various resolutions of the half-diminished chord demonstrate that:
The half-diminished chord (minor seventh with flattened fifth) is not new to Tristan und Isolde. It is part of the standard classical harmonic procedures. Its function was two-fold. Firstly, on the second degree of the scale it functioned as a secondary dominant, that is a preparation for the dominant or dominant seventh. Secondly, on the seventh degree of the scale it functioned as a substitute for the dominant seventh, often as a softer cadence than dominant to tonic.
Wagner's great leap forward in Tristan und Isolde is to liberate the chord. While it is still used on the second and seventh degrees of the key, it is also used on other degrees of the scale, primarily, but not only, on the flattened second, flattened sixth and sixth degree of the scale. It also takes on a life of its own, providing a harmonic colour to be heard and enjoyed in its own right, without the need for resolution.
Having completed Tristan und Isolde, Wagner returned to complete Der Ring des Nibelung and continued to employ the half diminished chord in Siegfried and Götterdämmerung, as he had done in the earlier parts of Der Ring des Nibelung, namely, Das Rheingold and Die Walküre. Its function in the later works tends to revert to its use as a secondary dominant on the second and seventh degrees of the key, where a key is established. As in the earlier works, key is often established by the use of supertonic and dominant harmonies without resolution to the tonic. The half diminished chord is not found in Parsifal or Die Meistersinger von Nürnberg (other than when Wagner quotes Tristan und Isolde in Die Meistersinger von Nürnberg) as the drama of these works require quite different harmonic sound worlds.
A detailed harmonic analysis of the Prelude to Act 1 of Tristan und Isolde can be viewed on YouTube