In 1614, John Napier discovered the logarithm which made it possible to perform multiplications and divisions by addition and subtraction.
(i.e.: a*b = 10^(log(a)+log(b)) and a/b = 10^(log(a)-log(b)).)

This was a great time saver but there was still quite a lot of work required. The mathematician had to look up two logs, add them together and then look for the number whose log was the sum. Edmund Gunter soon reduced the effort by drawing a number line in which the positions of numbers were proportional to their logs.

The scale started at one because the log of one is zero. Two numbers could be added by measuring the distance from the beginning of the scale to one factor with a pair of dividers, then moving them to start at the other factor and reading the number at the combined distance. Soon afterwards, William Oughtred simplified things further by taking two Gunter's lines and sliding them relative to each other thus eliminating the dividers.

A History of the Slide Rule
How To Use the Slide Rule