The promptuary, also known as the card abacus is a calculating machine invented by the 16th-century Scottish mathematician John Napier and described in his book Rabdologiae in which he also described Napier's bones. It is an extension of Napier's Bones, using two sets of rods to achieve multi-digit multiplication without the need to write down intermediate results, although some mental addition is still needed to calculate the result. The rods for the multiplicand are similar to Napier's Bones, with repetitions of the values. The set of rods for the multiplier are shutters or masks for each digit placed over the multiplicand rods. The results are then tallied from the digits showing as with other lattice multiplication methods. The final form described by Napier took advantage of symmetries to compact the rods, and used the materials of the day to hold system of metal plates, placed inside a wooden frame. Design of the Promptuary The promptuary consists of four parts: a set of number strips, engraved with a large digit at one end and with many small digits along the strip a set of mask strips, which Napier called excised or perforated strips. Each has a single digit engraved at one end and various triangular holes cut in it a board on which to place the strips when performing a calculation a box to store the strips. In Napier's design, the top of the box was the board on which the calculations were performed. The dimensions of the strips depends on the maximum number of digits in the numbers to be multiplied. For a device capable of multiplying two N-digit numbers together, the strips should be (N+1) times as long as they are wide, and there should be 10N number strips and 10N mask strips. So for example, for a promptuary capable of multiplying two five-digit numbers together, the strips should 6 times as long as they are wide, with 50 number strips and 50 mask strips. Napier's example specified strips 1 finger (19mm) wide and 11 fingers (209mm) long, enabling the device to multiply two 10-digits numbers to produce a 20-digit result. Napier specified that the number strips should be a different thickness from the mask strips - one quarter of a finger (5mm) versus one eight of a finger (2.5mm). This is not necessary for the operation of the device. The promptuary contains a lot more pieces than a