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. 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.

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 (some systems use a reversed flat sign instead), 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.

...
Wikipedia