Frequently in African music two or more ostinatos moving contrapuntally are employed, with or without a longer melodic line to create an orchestral texture (dense textures are desired and aimed for by both composers and performers alike). This type of polyphony is of the contrapuntal or horizontal type. In practice each ostinato moves in independent melodic and rhythmic patterns. Chords are normally formed using one of two techniques: the span process or scalar clusters. These chords can be embellished as a result of variation in which any combination of notes permitted by the scale can be used in a chord. However, in common practice, chords are formed by harmonizing in 3rds, 4ths, 5ths, 6ths, etc. The type of chord formed depends on the scale system being used. Recent research has shown that African music has chord progressions. Gerhard Kubik states "until recently, little attention has been paid to a further structuring element, namely, the tonal-harmonic segmentation of a cycle. In most African music, cycles are sub-divided into two, four or eight tonal-harmonic segments." (A theory of African music, Volume II, page 44, paragraph 5). In addition, the use of the tritone interval for tension is common in Africa. Oluwaseyi Kehinde notes "it is interesting that the interval of the tritone (augmented fourth or diminished fifth) is a salient feature in both vocal and instrumental music throughout Africa” (Karlton E. Hester and Francis Tovey use the same phrase to describe it). Gerhard Kubik in his article "bebop a case in point: the African matrix in Jazz harmonic practices" and his book "Africa and the Blues" echoes this point. This is significant to chords used as reference points or chord progressions in African musical structures. Through the use of parallelism cadential patterns are inevitable. O.O. Bateye clarifies: "The subdominant (plagal) cadence is (resulting from the frequent tendency toward parallelism in African music) the favored cadence and not the perfect cadence, which is the norm in classical western music...Cadential patterns are frequent in African music and invariably result as a consequence of melodic movement either by thirds, fourths, or fifths – that is as a consequence of what may be referred to as shadow harmony ... A cadential descending minor third is frequently noted between the minor third step and the tonic (Reiser, 1982:122) in African music."