ABSTRACT
A great deal of improvised keyboard music must have consisted of variations built on the repetition of a tune, or a melodic ground, or a harmonic ground. Since a tune and a melodic ground both imply a more or less fixed set of harmonies, one basis of variation easily merges into another. The variety of the pieces William Byrd gathered together in My Ladye Nevells Booke is however a strong indication that he saw no hard and fast distinction between those that sound well on one kind of keyboard instrument and those that sound well on another. The origin of the free keyboard fantasia, probably at first an improvised genre, lay partly in music based on plainsong melodies. Byrd's little two-voice fantasia, called 'a verse' by Thomas Tomkins, is unrestricted by a cantus firmus, but it does not differ widely from his two-voice settings of Clarifica me and Gloria tibi trinitas, and is likely to belong to the same period.
