The number line is a very simple mathematical idea.  In the simplest terms, it is a line with numbers marked on it at specified intervals.  Where the numbers are marked depends on the use of the number line.

Number lines may be used to plot or graph solutions to equations, by using two number lines at right angles to each other we can plot points and create maps.

A further example is the timeline where events can be marked according to the year or time they occurred.

When we look at a slide rule, we see that we have something similar ... a straight line with numbers marked on it.  Now this number line is a little different if we look at a one, say part of the C scale

we see that this is not like a normal number line. See how the numbers are not separated by the same distance.  This type of spread is called non-linear.

Looking at another scale, the L scale

we can see that the numbers are evenly spaced.  This type of spread is called linear.

As with any number line, the position of the numbers on the line (known as the scale) depends on what the number line will be used for.  In the case of a slide rule, the many scales are used for different operations.  We will be using the different scales later and see what they do.