Traditional sub-Saharan African harmony is a music theory of harmony in sub-Saharan African music based on the principles of homophonic parallelism (chords based around a leading melody that follow its rhythm and contour), homophonic polyphony (independent parts moving together), counter-melody (secondary melody) and ostinato-variation (variations based on a repeated theme). Polyphony (contrapuntal and ostinato variation) is common in African music and heterophony (the voices move at different times) is a common technique as well. Although these principles of traditional (precolonial and pre-Arab) African music are of Pan-African validity, the degree to which they are used in one area over another (or in the same community) varies. Specific techniques that used to generate harmony in Africa are the "span process", "pedal notes" (a held note, typically in the bass, around which other parts move), "rhythmic harmony", "harmony by imitation", and "scalar clusters" (see below for explanation of these terms).