In "the old days" - before the time of electronic pocket calculators and computers - there was still a need for calculations, that were too advanced/difficult to be made in the head or on paper.

For this you used mechanical calculators - calculators that work without electrical power and electronic circuits.

Basicly there exist two types of mechanical calculators: Those that could only add and subtract, and those that could also multiply and divide.

An example of the first type is Comptometer, which is a full-keyboard (9 keys for each digit) key-driven calculator. This means that the calculation is performed each time you press a key.

An example of the other type is Odhner, which is a rotary pin-wheel calculator. This means that you first set all digits and then uses a crank to perform the calculation.