Napier's Bones, or Napier's Rods, are certain instruments invented by Lord Napier, for performing some of the fundamental rules of arithmetic, by an easy mechanical process. They may be made of bone, ivory, horn, wood, pasteboard, or any other convenient material. There are five of them, and the face of each is divided into nine equal parts, each being subdivided by a diagonal fine into two triangles. In these compartments or squares the numbers of the multiplication table are inserted, the units or right-hand figures being placed in the right-hand triangle, and the tens in the left.