Now let’s connect some of these tetrachords to form basic shape of our first DASTGÂH: MÂHUR
If we connect the tetrachords DARÂMAD e MÂHUR and RÂST eith C as their common note (connected) and FINALIS of both tetrachords , we will have:
G A B C
C D E F
OK then! It looks like a C major scale with its root in the middle( like the Plagal modes)! But if we continue the scale, we should connect another RÂST with F as FINALIS and their common note , we will have:
G A B C D E F
F G A B♭
And then we can add a NAVÂ with C as FINALIS( in a Guše called ARÂQ), without using the common notes(not connected and with a whole step apart), the result will be:
G A B C D E F G A B♭
C D E♭G
And yet I haven’t inserted the KORON notes that we use for alteration, for example in guše*s like DELKAŠ and RÂKs( there are 3 kinds of them)and …
So I think now you know why we don’ use SCALES and it should be tetrachords.
And the end, you need to know that there is another kind of tetrachords connection(that will be discussed very soon.
* Gūše is a written melody that masters were using them to teach their students and in fact a DASTGÂH or ÂVÂZ, contains lots of Gūšes that could be played one after another.