One of the tools that demographers use to understand population is the age structure diagram (it is sometimes called a population pyramid, but it is not always pyramidal in shape). This diagram shows the distribution by ages of females and males within a certain population in graphic form. Figure 1 shows a diagram in which the ages and sexes for the United States population are arranged so that ages are grouped together such as 0 – 4 years, 5 – 9 years, and so on. The population of each is group is represented as a bar extending from a central vertical line, with the length of each bar dependent upon the population total for that particular group. The centerline separates the females from the males. The female and male populations for each group are represented by the distance from the centerline, with females on the right and males on the left.