How is the straight-line distance measured on a map?

The straight-line distance on a map can be easily measured using a strip of paper with straight edge in the following way.

1. First mark the points, say A and B, between which the distance is to be measured
2. Place the edge of the strip of paper along the line AB.
3. Mark the limits of distances on the paper.
4. Now place the paper along the linear scale and read the map distance.
5. The distance between points A and B can also be measured using a divider.