A slide rule uses logarithmic scales, where multiplying two numbers means adding their logarithms. Slide one scale against the other, line up the values, and the ruler does the multiplication geometrically.

I didn’t know it worked with logarithms! Cool!!!

Only boomers & older folk will know what slide time is. Anyone born aft 1970 wouldn't possibly have bought or seen one cuz calculators killed slide rule industry in early 70s.

A slide rule gives only the sequence of significant digits. It does not indicate the position of the decimal point or the order of magnitude. You must track that yourself mentally.

When calculators (ex HP35) appeared, slide rule use essentially disappeared. For those of use who had bad eyes, it was good riddance.

Works based on the fact that the log of a product equals the sum of the logs. Can also be used to divide, since the log of a quotient equals the difference of the logs. Notice if you look at the numbers on the top over the numbers on the bottom, you get the "2" that appears under the "1." Example: 4 over 8 equals 2.