set of Napier's Bones. A set of Napier's Bones with 20 rods is capable of multiplying numbers of up to 8 digits. An equivalent promptuary needs 160 strips. In the examples and illustrations below, N is set to 5 - that is, the illustrated promptuary can multiply numbers of up to 5 digits. Number Strips The number strips are divided into five squares, with about half a square free at each end. A large digit, known as the simple, is marked in the space at the top of the strip. A multiplication table is placed in each of the five squares. Each of these multiplication tables is identical—it lists the multiples of the simple and is laid out in a particular way: The square is divided into nine smaller squares in a three-by-three arrangement. Each of these is divided into two triangles by a diagonal line running from lower left to upper right. The multiples of the digit at the top of the strip, the simple, are marked in the table as in the diagram. The simple itself is marked in the triangle labelled × 1. The number that is two times the simple is marked in the two triangles marked × 2. If the number is two digits, the first digit is placed to the left of the main diagonal (marked in red) and the second digit to the right of the diagonal. If the number is a single digit, it is marked to the right of the diagonal The number that is 3 times the simple is written in the triangles marked × 3 in the same way as the multiple of 2 The other multiplies of the simple are marked in the other triangles in the same way. Zeroes may be written in or left blank. This does not affect the operation of the device. The triangle in the bottom left of the table is always blank. The following diagram shows the multiplication table for the simple 7: Complete number strips for the simple 7 and the simple 2 are shown in the following diagram. The lines delineating the triangles have been omitted. The Mask Strips Mask strips are placed horizontally across the calculating board, that is, from left to right rather than from top to bottom. They have a large digit written in the space at one end and the rest of the strip contains five