"Las Vacas" is a set of variations, based loosely on the renaissance ground bass by the same name. The opening states the descending bass line, starting on A and finally ending on D, but after a few variations an F appears under the opening A, changing the tonality. Yet a few variations later, a B-flat is put under the F and A, leading to a chord that is anachronistic by several centuries. The final variations turn the bassline upside down, going from D up to A.
While the chord structure of this piece is sometimes far removed from the renaissance idiom, the variations end almost invariably on open fifths, maintaining the archaic feel.