Consider the difference between the standard procedures for measuring length and area. Length is measured directly. The length of a segment is the number of arbitrarily chosen unit lengths that fit end to end on the segment. We measure length by using a ruler-like device that has the unit lengths already marked off and counted on it. Area, in contrast, is measured indirectly. Although the area of a region is the number of arbitrarily chosen unit areas that cover the region with no overlap, almost always we measure area by taking linear measurements and inserting them into a formula.