A note on Persian music:

Traditional classical Persian music is based on the dastgāh system, which organizes small melodic and motivic units into 12 different groups based upon the concept of modal progression. The concept of mode in Persian music includes both use of particular tones and arrangements of these tones, and also extra-musical associations such as mood and characteristic meaning, particularly on a mystical level. Each of these dastgāh-s have their own state they would ideally evoke; such as majesty, melancholic wisdom, or desire for reunion.

The ghazal, a form of lyric poetry, is the main poetic form used in the body of the vocal performance (āvāz) of the dastgāh. It unifies the music thematically and rhythmically. The major portion of the āvāz has no musical meter, but instead is organized rhythmically by the poetic meter of the ghazal. The poetic meter is based on syllable length, which is condensed or expanded according to musical interpretation.

The musical form is also organized around the poetic form. The ghazal consists of five to seventeen couplets, some of which are chosen by the singer for his performance. The musical phrase follows the poetic phrase, and is organized into distich (beyt), hemistiches (mesrā), and internal phrases. The phrasing is usually symmetrical and according to poetic feet. These phrases are defined by pauses in the music. The poetic line itself is usually preceded and followed by wordless vocalization.

The ghazal uses monorhyme, with the second mesrā of each couplet rhyming: aa/ba/ca/da. Musically, there is often a cadence at the end of the rhyming mesrā, a return to a consistent reference pitch. Each non-rhymed portion often initiates a new pitch level. These new pitch levels are usually associated with different pieces (gushe) within the dastgāh, and each is usually in a different mode. Thus, for each couplet or two there is a different musical piece used to express it. These pieces follow each other in a specified order designed to create the desired effect on the listener.

(from Melodic Contour in Persian Music, and its Connection to Poetic Form and Meaning, 1986, by Margaret Caton, Ph.D.)