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Sterling Currency (£sd) Multiplication on a 12-key Add-Lister

Instructions for performing multiplication on Sterling currency (£sd) values using a mechanical 12-key Add-Lister.

This is a new article and was not in "The International Calculator Collector".

These instructions are taken from an article in the journal Office Magazine in 1956[1].  They illustrate one of the more complicated techniques, performing multiplication of non-decimal £sd currency amounts using a simple mechanical adding machine.



Britain's Sterling (£sd) money system before 1971 was not decimal, but was based on the centuries-old Pound, Shilling, & Penny, where :-

                12 Pennies (Pence) = 1 Shilling

                20 Shillings = 1 Pound

So 1 Pound = 240 Pence

This may seem a little strange now, but when taught from a very early age it came naturally.  Also it did have a few advantages; for example it was easy to split one Pound exactly between 3 people - each one would be given 6 Shillings and 8 Pence - also 1 pound could be split exactly into 4, 5, 6, 8, etc.

The way of writing a price in this system was easy - for example, a sum of money comprising 21 Pounds 15 Shillings and 11 Pence would be written £21 15s. 11d. (or £21/15/11).

Several other countries, mainly in the British Commonwealth, had also used a similar monetary system but all changed to decimal systems by 1972.

As can be imagined, this non-decimal system caused difficulties when calculating, especially multiplying and dividing.  It was possible to use a standard decimal calculator by using tables to convert from Shillings and Pence to fractions of a Pound and vice versa.

However, special versions of the simpler calculators (Comptometers, adding machines, add-listers, and addiators) were made which worked in Pounds, Shillings, & Pence.  Some of these calculators are featured in the featured Non-Decimal Calculators section of this site.

Such an add-lister is the hand-operated Olivetti Summa Prima 20, shown below.  The decimal version of this model would be called a "10-key add-lister", since it has 10 number keys.  The £sd currency version here has two extra keys, marked '10/12' and '11/12', to enter 10 pence and 11 pence, respectively, so it can be called a "12-key add-lister".

Olivetti Summa Prima 20 £sd

Olivetti Summa Prima 20 £sd currency add-lister.  Note the extra number keys labelled '10/12' and '11/12' for entering 10 pence and 11 pence, respectively.

The following article explains how to multiply an amount in the £sd currency using such a machine.


"Sterling Multiplication on an Adding-Lister

    Some readers whose offices are equipped with an adding-lister little realize how much useful sterling multiplication can be done on them.  The required technique may appear complicated, but it is soon mastered after a trial run or two, during which one can mentally follow the work being done by the machine.

    For multiplication, whether decimal or sterling, it is of course essential for the adding-lister to be fitted with both a repeat key [the black switch with the 'X' above it, on the Summa Prima 20, above, is moved upwards] and a non-add key [the white control knob of the Summa Prima 20 is moved to the left].

    As an example let us take 43 times £769 17s. 6d. [769 pounds, 17 shillings, and 6 pence] and work it out on an Olivetti Summa 15 [an earlier model than the Summa Prima 20 but with similar functionality], a 10-key hand-operated machine.  On a full keyboard machine or an electrically-powered one, exactly the same principles apply.

    First we put the repeat key to "repeat" and set the amount (£769 17s. 6d.) on the keyboard.  The operating handle is then pulled three times to correspond to the units digit in the multiplier (43).  The keyboard is then cleared.

    The next step is to multiply the multiplicand by 10: copying from the tally roll, one sets 769 on the keyboard; then the shillings in the multiplicand are divided by 2 (i.e. 17÷2=8 and 1 over) and both the result and the remainder are set on the keyboard in that order.  Had there been no remainder the cypher key [black key with one white spot, to insert 0 in a column] would have been tapped once.

Table for the Pence

    We now have set on the keyboard 76981, but we have not finished, for the pence column remains.  Here we multiply the pence by 10, convert the result to shillings and set the amount on the keyboard.  In our example it is 10x6=60d.=5s.  Since there are no pence left over, the cypher key has to be tapped once.  (For operators who dislike having to convert mentally a large number of pence into shillings, a conversion table is easily typed out on a piece of card.)

    We now have in the keyboard the following amount £7,698 15s. 0d. (our original figure multiplied by 10).  To multiply by 40 merely means that the operating handle has to be pulled four times.

    All that remains to obtain a final total is to clear the keyboard, return the repeat key to normal, put the total key to total and pull the operating handle, when the result (£33,104 12s. 6d.) will be printed on the tally roll.

By Three or More Digits

    Had the multiplier been 243 we could have carried the calculation a stage further, multiplying again by 10 and then pulling the operating handle twice.

    Copying again from the printed strip one sets ten times £7,698 15s. 0d. by pressing the keys 76987100; the operating handle is then pulled twice.  The keyboard is cleared, the repeat key is put to normal, and the total key pressed; a final pull on the operating handle will then throw out the result £187,079 12s. 6d.

    Had the multiplier been 203, the multiplicand would still have to be multiplied by ten for the tens column of the multiplier but the answer instead of being put into the machine would be printed only by the non-add/print key.  The next stage, that of multiplying again by ten to obtain the correct figure for the hundreds can then be started by reading off from the tally roll.

    The basic rule for tens multiplication is to re-enter the pounds, halve the shillings and call them pounds and multiply the pence by ten.  The important thing to remember is that the cypher key must be tapped if there is no remainder to the two mental calculations."


After reading these instructions I feel very glad that Britain, and the other countries, changed to decimal currency.

The only electronic calculator with any Sterling currency (£sd) functionality is the ANITA Mk10, described on the sister "ANITA Calculators" site, and even then its capability is somewhat limited.



  1. "Sterling Multiplication on an Adding-Lister", Office Magazine, January 1956, p32.

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Text & photographs copyright, except where stated otherwise, © Nigel Tout 2000-2024.