Arab tone system
The modern Arab tone system, or system of musical tuning, is based upon the theoretical division of the octave into twenty-four equal divisions or 24-tone equal temperament (24-TET), the distance between each successive note being a quarter tone (50 cents). Each tone has its own name not repeated in different octaves, unlike systems featuring octave equivalency. The lowest tone is named yakah and is determined by the lowest pitch in the range of the singer. The next higher octave is nawa and the second tuti. (Touma 1996, p.17-18) However, from these twenty-four tones seven are selected to produce a scale and thus the interval of a quarter tone is never used and the three-quarter tone or neutral second should be considered the characteristic interval. (ibid, p.23)
By contrast, in the European equally tempered scale the octave is divided into twelve equal divisions, or exactly half as many as the Arab system. Thus the system is written in European musical notation using a slashed flat for quarter flat, a flat for half-tone flat, a slashed flat and a flat for three-quarter tone flat, sharp with one vertical line for quarter sharp, sharp (♯) for half-step sharp, and a half sharp and a sharp for three-quarter sharp. A two octave range starting with yakah arbitrarily on the G below middle C is used. (ibid, p.24)
In practice far fewer than twenty-four tones are used in a single performance. All twenty-four tones are individual pitches differentiated into a hierarchy of important pitches, "pillars", which occur more frequently in the tone rows of traditional music and most often begin tone rows, and scattered less important or seldom occurring pitches (see tonality). (ibid, p.24-25).
The specific notes used in a piece will be part of one of more than seventy modes or maqam rows named after characteristic tones which are rarely the first tone (unlike in European-influenced music theory where the tonic is listed first). The rows are heptatonic and constructed from augmented, major, neutral, and minor seconds. Many different but similar ratios are proposed for the frequency ratios of the tones of each row and performance practice, as of 1996, has not been investigated using electronic measurements. (ibid, p.18) Actual practice is estimated to vary within a quarter tone from notation.
The current tone system is derived from the work of al-Farabi (d. 950 CE) (heptatonic scales constructed from seconds) who used a 25 (unequal) tone scale (see tetrachord), with Mikha'il Mishaqah (1800-1889) first presenting the 24 tone equal tempered division. (ibid, p.19)
• Habib Hassan Touma (1996). The Music of the Arabs, trans. Laurie Schwartz. Portland, Oregon: Amadeus Press. ISBN 0-931340-88-8.