The wood, of course. Using x-ray images taken from multiple different angles, radiologist Berend Stoel of Leiden University Medical Center in the Netherlands proved that the spruce and maple wood used in five violins made either by Antonio Stradivari or Giuseppe Guarneri del Gesù—the rival master luthiers of Cremona—had fewer variations in their density than that in seven contemporary violins. The density of the wood determines how a violin resonates with sound, which may explain why Stradivarius and Guarnerius violins are coveted by musicians worldwide and fetch prices of several million dollars. It may also allow modern instrument makers to finally match the perfection of past masters.