Mechanical devices for multiplying and dividing real numbers. At its simplest, a slide rule is just 2 scales of positive numbers located at distances from 1 proportional to their logarithms. The 2 scales are arranged on opposite sides of a sliding rod, so they can be moved relative to each other.

1       2   3   4 5 6
        1       2   3   4 5 6
The diagram (distorted scale, to make everything line up nicely) shows how a slide rule is used. The bottom scale is moved so the `1' is lined up against the multiplier `2'. In this position, any number on the bottom scale lines up with itself multiplied by the multiplier. Of course, this also means we can divide by the multiplier by going from the top scale to the bottom.

Slide rules typically have another pair of scales with all distances halved. This not only gives greater range (at the cost of precision), but also enables you to square and take square roots, simply by transfering a number from the half-distance scale to the full-distance one.

More functions are often found on the back of the slide (you take it out, flip it over, and insert it like that) -- special scales can give you logarithms of sines, cosines, and tangents; secants are trivially there, since multiplication is easy on a slide rule.

Precision is typically a bit more than 2 significant digits, if you're careful -- more than enough for most engineering tasks.

Maintenance instructions: Keep slide rule in box, away from any moisture which might warp the scales. If slide sticks, remove it and apply talcum powder.