Slide rule

Slide rules were used to do basic arithmetic operations, at least until electronic calculators came up in the 1970's. Most of them had additional scales that could be used for calculations with logarithmic values, trigonometric functions or roots. The exakt type of a scale is indicated by the little letter at the slide rule's left side, a red letter indicates a scale with descending values. The additional slide could be used to read values more precisely.

In addition to the regular slide rules, like the one shown here, there were special types with additional scales for usage in e.g. engineering. Pilots or engineers sometimes even wore wristwatches with circular slide rules, that still can be bought nowadays.