A rather abridged quote from Wikipaedia:-
These medieval portative organs, so extensively used during the 14th and 15th centuries ..... contains nine pipes..

A mode is not constrained to start on any particular pitch, it is just a sequence of intervals, like our modern major and minor scalea. The names we give to notes are just an administrative convenience.

So it may be (tho' others may well know better) that every mode would start on the lowest note of the keyboard, and these primitive instrunents had to be retuned each time the mode changed, thus keeping the pitch at a comfortable level for the singers. (A reversal of the process whereby guitarists use a capo to change pitch without changing "mode", to suit a voice with a different range). A good reason to use one mode for a month!