Hatching is a drawing and printmaking technique in which closely spaced parallel lines are used to indicate tone, shadow and volume. Cross-hatching adds a second layer of lines running perpendicular to the first, increasing the density and darkness. The technique is fundamental to engraving, etching, and pen-and-ink drawing, where continuous tone is impossible and gradations must be simulated through density of line. Albrecht Dürer was perhaps its greatest master in printmaking: in his engraving Knight, Death and the Devil (1513), the entire tonal structure — the chiaroscuro of the armour, the shadow on the horse's flanks — is built from systems of hatched lines that follow the forms they describe. In pen drawing, Leonardo's hatchings in his anatomical studies follow the contours of muscles and bones, making line itself a form of description. Cross-hatching appears in virtually every medium where line is the primary element.