The SLIDE Rule
The Slide rule could be 6" to over two feet long, and, in the example above, the light blue "board" (the slide) is fitted "tongue in groove" in the main body (the "stock") of the Slide Rule, and slides easily left and right, so numbers on the "slide" of the rule can be aligned with numbers on the stock of the slide rule, either above or below the slide. The clear plastic window (the cursor) also slides, and the red vertical line (hair line) is used to more accurately line up the numbers to yield the mathematical answer. Can you even GUESS how to use the slide rule?
On the stock and slide there are marked a number of scales. In the simplest slide rules there may be two scales on the stock and two on the slide. On a complex rule there may be a total of twenty or more scales. Some rules have scales on one face only and a cursor which can only be used on that face; these are referred to as "simplex" rules. Others have scales on both faces with a cursor which can also be used on both faces; these are referred to as "duplex." A very common intermediary between the two forms is found on rules with scales only on one face of the stock but on both sides of the slide. Within this latter group there are two variants; usually a small mark on the back of the stock allows the slide to be used without removing it but some times the slide has to be removed and inserted with its face reversed.
Another implementation of the Slide Rule is the CIRCULAR or ROUND Slide Rule.
Scientists and mathematicians routinely used the slide rule for complex calculations.
(The webmaster actually used one in high school, though, truth be known, could not even give you a CLUE as to how to compute with one!)