Promptuary

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 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.

A set of Napier's Bones with 20 rods is capable of multiplying numbers of up to 8 digits.

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.

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 squares.

So for example the mask strips for the simples 3, 6 and 9 will look as follows: The guide lines in the patterns are for positioning the holes.

The result of the multiplication is read from the device by examining the digits visible through the triangular holes in the mask strips.

The multiplication process required only addition, and no intermediate results needed to be written down.

[3] It is not known who was the maker of this piece, nor if it is of Spanish origin or came from a foreigner, although it is probable that it originally belonged to the Spanish Academy of Mathematics (which was created by Philip II) or was a gift from the Prince of Wales.