•     The slide rule was invented around 1620 - 1630.
•     Many people still use slide rules to this day, with a little practice you can perform calculations to a reasonable estimate very quickly.
•     The military in many countries still teach slide rules as a backup in case of calculator malfunction
•     Slide rules have even been used by astronauts. They have been used on 5 of the Apollo space missions.
•     Some people belive the advent of electronic forms of calculation to have tainted the art of engineering. eg. When computers were first introduced new engineers went to them to solve problems that experienced engineers could solve quickly with a few quick uses of a slide rule. In fact several computer centers at the time had a framed slide rule hanging on the wall with the message "In case of emergency, break glass".
•     Many people like to use a slide rule as a check to make sure they have calculated correctly on a calculator or computer. The slide rule will only be an estimate (though normally quite a close one) and will have to be done slightly differently, minimising the chances of duplicating mistakes.

Before electronic hand held calculators, the slide rule was widely used in Engineering, Science and Commerce for rapidly performing calculations involving multiplication and division which have to be accurate to not more than three or four decimal places. It can also be used for such operations as involution (raising to a power) and evolution (extraction of a root) and for calculations with trigonometric functions (sine, cosine, tangent, cotangent).
The interactive slide rule on this site allows people to understand how this tool was used.
To experience an Interactive Slide Rule hit the Virtual Slide Rule tab (above) and you will find an Interactive Slide Rule complete with instructions detailing the reading of the slide rule. There is also a brief history of the invention of the slide rule.