The subtraction version of Napier's Chessboard Calculator, like the additive version, identifies each column on the board by the binary positional value of the numbers to be represented in each row. Thus the starting stage is to enter the numbers to be added together in the rows of the board, either by clicking on the board cells or by entering the desired value in the textbox alongside each row. [**NOTE:** when the text box is used as the data entry method, click on the ENTER button before proceeding.] After data entry, row values may be changed either by adding or deleting chips or by re-entering the text box value. Only two rows can be used to enter data items - the upper pair of those identified with a text box to the right. The bottom row is to be used to trigger the row by row subtraction.
Once the data has been entered, click on READY, and then click on a cell in the bottom row. In those cases where a borrow is not necessary in the top row, the subtract action will be carried out. Where a borrow is necesary, click on the cell where there is a value to borrow from and the values will be distributed to the right. Single valued cells are symbolized by a red chip; where the number of chips in a cell is more than one, it is shown in purple. Be careful not to distribute borrowed chips too far left! The subtraction proces is complete when the second row (the subtrahend) is zero (or blank). The result will be shown alongside the upper row of the original rows of values.