It's famous saying that
there are 7Dastgah in modern music of Iran : Mahur , Shoor ,
Chargah , Homayon ,Segah , Nava and Rast Panjgah .
Dastgah is a song which doesn't resemble to any degree of scale
and its intervals. For example , when we say Mahur dastgah , we
mean that there are melody keys which include greater scale.
There are two songs in any dastgah major song and minor song.
both of them are in the first scale but with different notes
which starts from a note other than tonic and deals mostly with
other notes. minor song is taken from other Dastgah but it must
descend to basic Dastgah.
Iranian song are divided into 3part :
If you suppose song a country ; Dastgah is its province;
Naghmeh , city and Gusheh , house .
meets the requirement of having a right fourth and fifth feet and interval among
notes must be the same and shouldn't resemble to each other and it can
have Naghmeh and gusheh . For example , Shur , Mahur and etc .
Naghmeh , as we described it as a city , is one of main member of Dastgah ;
although it isn't possible to specify a special scale for it. The scale
changes the mother Dastgah because of shahed note and variable and stop ones . so Dange
change which it seems difficult to recognize.
Naghmeh can have many gusheh . Gilaki, kurd bayat, Dobeiti are Naghmeh gusheh
of Dashti. Hejaz is a gusheh of Abu ata and fili and ghatar are gushehs of
Bayate turk .
Gusheh is a song which is in 1or
1/5 of a dange and doesn't exceed from many notes
and there isn't any change in these notes .
so one can't specify a scale for it and gusheh doesn't have any smaller one. But
sometime shahed and stop notes are regarded .
It is a note which voices a lot in
Naghmeh and gusheh . In fact , it is axis of
a song . If you analyze a tasnif or a rang , you will see that pedal note
voices more another note.