Used for scientific and engineering calculations, a slide rule was an analog computer consisting of three interlocking calibrated strips, where the central strip could be moved lengthways relative to the other two, and a cursor with an alignment line which is used to align the scales.
The principal scales are calibrated logarithmically. By the skilled manipulation of these strips, multiplication and division can be performed by the mechanical addition and subtraction of logarithms.
Typically two significant figures of precision were possible, with three being obtainable by expert users.
Slide rules also had other mathematical functions encoded on other auxiliary scales.