Here is an excerpt from Introduction to the Study of Musical Scales by Alain Danielou. The important part is in bold. I included the rest of the passage only to provide context:
It is the loss of this pedal point of the tonic, somewhere in the course of ages, which renders the modes of plain-chant so vague and so weak. They lack a basis, and their classification becomes a rather abstract game, like the system of the Greek Doris! i in the unreal form generally attributed to it. It is by an artificial conception that a mode can be assimilated to the plagal 9 form of another mode. The melody remains unaffected and in the tone of C (Sa), because, to determine a tone, a permanent element is required, and habit can, up to a certain point, create this permanence in favor of C (Sa): that is, in favor of the western major mode, supposing that we are still using only the white keys of the piano. Another mode can be determined only by constantly imposing its tonic. Percussion instruments, such as drums, cymbals, etc., can be sufficient to determine this tonic. This is why, in ancient music, the problem did not arise, as such instruments were always present in a musical performance. The accurate tuning of the different notes of the scale should, of course, be sufficient to define the tonic, but, as the differences in intervals are very small, rare indeed are the musicians who can maintain this accuracy without referring constantly to the tonic.
Do cymbals as well as other “unpitched” drums actually have a pitch?
