According to contemporary theorists, Persian art music is organized into twelve systems called dastgah, a frequently used Persian word meaning "apparatus," "mechanism," "scheme," or "organization." Just as there is a dastgah for conducting government operations, for receiving radio programs, or for weaving rugs, there are dastgah-ha for making music.
The seven main dastgah of contemporary Persian art music are: Shur, Mahur, Homayun, Sehgah, Chahargah, Nava, and Rast Panjgah. In addition, there are five smaller systems which are considered auxiliary dastgah-ha. These satellite systems, four belonging to the dastgah of Shur and one to Homayun, are designated by the terms avaz (song) or naghmeh (melody). Their individual names are: Abu Ata, Bayat-e Tork, Afshari, and Dashti, associated with the dastgah of Shur ; and Esfahan, associated with Homayun.
Whereas a literal translation of the word dastgah as "apparatus" aptly describes the Persian musical system, Persian music theorists usually translate dastgah as "mode." The term mode does give the foreign musicologist a reference point. At the same time, however, it is misleading, because a Persian dastgah is more than a mode in the sense that the Western church modes are understood. Like a mode, each dastgah has its own seven-note scale, and some notes have special significance within that scale. But in addition, each dastgah has its own special repertory of melodies called gusheh-ha (singular gusheh), which embody the most characteristic aspects of the dastgah. When a performer is asked to define a certain dastgah, he does not play its scale, but its first gusheh.
To create a composition the performer puts several gusheh-ha together, usually following a traditional order that involves an increasingly higher range for successive gusheh-ha: the first few gusheh-ha fill the lower half of the octave; the next few fill the rest of the octave; and later gusheh-ha may rise to the beginning of the second octave. The differences in range of four important gusheh in the dastgah of Shur are shown in Example ι (the note shown as a quarter note receives the most stress in the melody).
The subdivision of the dastgah into gusheh-ha is also illustrated in the following diagram. Here the bottom horizontal line represents the lowest note of the octave and the upper line is the note one octave higher. Each rectangular unit is a gusheh. The dotted line that descends to the tonic to the right of the gusheh represents the forud (descent), a short melody frequently played at the conclusion of a gusheh to connect it to the parent dastgah. The melody of the forud is especially characteristic of the main dastgah and always returns to main cadence note of the dastgah.
Playing the gusheh-ha of a dastgah, or "going to its corners," as the process is described in Persian, is a technique somewhat analogous to modulation. As the different gusheh-ha are played, fresh tonal areas unfold and new accidentals may be introduced. The rise in tessitura during the successive gusheh-ha helps to increase excitement, which is only momentarily released by the occasional descent to the starting point in the forud.
Because the gusheh-ha are the musical materials used as models for improvised composition, each gusheh is actually more than what a Western musician thinks of as melody. What is important is not the gusheh as a tune but, in a sense, the gusheh as genetic materials for the creation of new pieces. The genetic materials provided by the gusheh are : the modal and rhythmic features of the melody, its shape, and other features of mood and character that may be the sum of the above plus other, indefinable ingredients, such as extramusical associations. Using this material, the performer improvises a number of small pieces that, when collected, form the gusheh, one of the movements of a dastgah composition.