Harmony emerged as music developed from monophonic [single-voiced] to polyphonic [multiple-voiced]. It is a powerful means for creating tension and resolution in music. In many genres of music, harmony is melody’s most important partner.
Harmony in the Renaissance era
In Renaissance music, harmony is often created through the superimposition of several melodies. This way of using multiple voices is called polyphony. The scales used to create these melodies are known as modes, slightly different from the major and minor tonality that became established later. Because this music is based on modes, we sometimes hear harmonies that sound strange or unexpected. At this time, there were strict rules determining which notes could sound at the same time in music; wrong combinations of notes were impure and hence the work of the Devil.
Harmony in the Baroque era
Harmony came to be thought of as chords that laid the foundation for the melody. Major and minor as we know them today became established as keys instead of modes, and the harmonic patterns of tonality began to evolve. This became such a fundamental element of Western music that even today we expect chords to follow each other in a particular order in order for the music to sound ‘familiar’. Basso continuo, meaning an accompaniment for melodies with harmonic accompaniment improvised on the basis of a bass line furnished with numbers indicating the chords (thorough bass, similar to the chord symbols used in popular music today), evolved at this time. The basso continuo group became the core of the orchestra, and orchestras were usually led by the basso continuo keyboard player and not a separate conductor. This was the origin of the clear separation into melody, bass and harmony that survives to this day in many genres of music.
Harmony in the Classical era
The Classical era inherited the structure of homophony, or melody + accompaniment, from the Baroque era, but instead of the ornaments and profusion of the Baroque, the goal was now to achieve a systematic, natural and simple expression in both the melodic lines and in the harmonies underpinning them. For the first time in the history of music, theoretical rules prescribed how harmonies should progress, and the functional harmony of tonality was established. Composers either followed these rules or, somewhat later, began to break them – and breaking the rules of harmony prompted reactions in listeners, forcing them to pay attention to what the composer had to say. Another difference between the Classical and Baroque eras was that now all accompanying parts were written out instead of providing a numbered bass line (figured bass or thorough bass) for a standardised basso continuo accompaniment.
Harmony in the Romantic era
Harmony became more complex, adding chromaticism, increased use of semitones and unusual progressions, and in general going beyond the Classical rules concerning the progression of keys and harmonies. The opera Tristan und Isolde by Richard Wagner is commonly held as one of the first steps towards the dissolution of tonality, i.e. the practice of music having a clear anchor point, or tonic. Early signs of pushing the boundaries of tonality may also be found for instance in the opera Salome by Richard Strauss.
Harmony in Impressionism
Composers began to use harmony like colour. Different chords could coexist and did not need to be resolved as usual. Chords either create tension or resolve it, and generally we expect a tension-creating harmony to be resolved, but Impressionism consciously worked against these expectations. The harmonies here include dissonances or vertically broad chords of stacked thirds (with ninths and elevenths), whole-tone scales and pentatonic scales, and harmonies made up of fourths and fifths.
Harmony in Expressionism
Instead of the seven-step scale of traditional tonality, we have here the twelve-tone technique or dodecaphony, where all 12 pitches of the chromatic scale are considered equal. The way for composers to organise this material is to devise a tone row determining the order of the 12 pitches. The basic rule in using this technique was that every pitch had to be used, in order, before any of them were repeated. The purpose of this was to prevent the ear from perceiving any note as the tonic, or fundamental pitch.
Harmony in the modern era
A composer may approach harmony from any number of angles. They use noises, microintervals smaller than a semitone, sound fields, spectral sounds. Non-musical sounds are often used as part of a soundscape – or all of it.