In the Preface to his Leçons de ténèbres, Couperin mentions the possibility
of transposing them to suit other voices. The first of these three pieces
has sections in a rather large span of keys, since parts of it are written
in D major, and others in F minor (but with only 3 flats in the signature),
among other keys (D minor, F major). What would be the possible (downward)
transpositions if one wants to remain within the bounds of the then current
temperaments? I don't see anything else but a 4th below (which would give
A major to C minor for the two extremes).