Although melodies are sometimes heard on their own, they will often have an accompaniment played by different instruments, or combinations of instruments, which may be partly determined by genre or convention. For example, convention in classical music might determine that a cello is accompanied by an orchestra when performing a cello concerto. In popular music, we might expect that a singer is accompanied by a lead and rhythm guitar, bass guitar and drums.
The accompaniment does not restrict itself to the pitches used by the melody but combines a number of different pitches to form harmony. Although harmony may not appear to be the main focus of a piece of music, particularly where a melody is prominent, it provides an important foundation which ultimately determines pitch choice and the musical journey which is taken.
Harmony can often be reduced to a series of chords which are formed from two or more pitches sounded together. This movement of chords is described a chord progression or harmonic progression. This forms the foundation described above, and can be analogous with a journey. This musical journey uses the concept of a ‘home’ chord to orient the listener; we are aware when the music departs from the home chord, and aware when the music returns home (giving a feeling of completeness).