"Passamezzo" is a series of variation on the Passamezzo Moderno ground bass.
The Passamezzo Antico and the Passamezzo Moderno were popular ground basses (in modern parlance: chord progressions) in the renaissance. This piece is based on the Antico bass, which is in minor; the Moderno is a corresponding variant in major.
This composition consists of 14 variations of the Passamezzo bass, divided in four sections. With the exception of the short 3rd section, in the style of a French overture, all sections build up gradually, and end with a variation that has a virtuoso soprano part. For the rest there are no sopranos to be found in this piece.
Of course, having been written in modern times, this piece is only pretend-renaissance, and in fact uses some anachronisms, such as the equivocation between V and IIb. Note in rehearsal letter M another thing that would never occur in the renaissance: a great bass solo.