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Flamenco music is melodically organized around the natural scale in E, the so called phrygian mode according medieval terminology or dorian mode according to the old greeks. This scale was most important in old times in the mediterranean area, and to this day it is part of musical traditions in north Africa and the middle east.
Generally the phrygian mode is conceived as a descending scale: E-D-C-B-A-G-F-E. The last four notes of this sequence form the basic tetrachord in flamenco music: A-G-F-E. On top of each of these notes a chord is built: A minor-G major-F major-E major, these are the four basic guitar chords that have been accompanying flamenco singing and dancing for centuries.
In order to accompany a flamenco piece with a melodic udu, the thing to do is use an udu whose two basic notes (beating hole closed/beating hole open) match two of the notes of the phrygian tetrachord being played. For example, if the mode was E phrygian then an apropriate udu would be the E3-A3 udu. But it is not mandatory that both basic notes of the udu are part of the phrygian tetrachord, it’s also good if both notes are part of the phrygian scale, like the A-D udu (low or high), the G3-C4 or the B2-E3. At least we need that just one basic note is part of the phrygian tetrachord, like in the F3-Bb3 udu.
The same principle applies to play any music. Any melodic udu can play any scale, but the udu’s basic notes should match notes of the scale being played. For example, a song in C major can be accompanied with udus tuned in C3-F3, B2-E3, A-D(low or high), E3-A3 and D3-G3.
Also remember that a pitch shifter can make an udu to play in different notes.