The slide rule, also known colloquially in the United States as a slipstick, is a mechanical analog computer. The slide rule is used primarily for multiplication and division, and also for functions such as roots, logarithms and trigonometry, but is not normally used for addition or subtraction. Though similar in name and appearance to a standard ruler, the slide rule is not ordinarily used for measuring length or drawing straight lines.
The Reverend William Oughtred and others developed the slide rule in the 17th century based on the emerging work on logarithms by John Napier. Before the advent of the pocket calculator, it was the most commonly used calculation tool in science and engineering. The use of slide rules continued to grow through the 1950s and 1960s even as digital computing devices were being gradually introduced; but around 1974 the handheld electronic scientific calculator made it largely obsolete.

In its most basic form, the slide rule uses two logarithmic scales to allow rapid multiplication and division of numbers. More elaborate slide rules allow other calculations, such as square roots, exponentials, logarithms, and trigonometric functions.

Most people find slide rules difficult to learn and use. Even during their heyday, they never caught on with the general public. Addition and subtraction are not well-supported operations on slide rules and doing a calculation on a slide rule tends to be slower than on a calculator.

This my one from my school days, I didn’t understand it then, and I still don’t today 🙂