Double pipes, such as those used by the ancient Greeks, and ancient bagpipes, as well as a review of ancient drawings on vases and walls, etc., and ancient writings (such as in Aristotle, "Problems," Book XIX.12) which described musical techniques of the time, indicate polyphony. One pipe in the aulos pairs (double flutes) likely served as a drone or "keynote," while the other played melodic passages. Instruments, such as the seven holed flute and various types of stringed instruments have been recovered from the Indus valley civilization archaeological sites.