A set of two histograms set on a vertical axis and back to back which depicts the numbers of the two sexes in different age groups. Males are usually on the left with females on the right. The youngest group is at the base; the oldest group at the apex. Actual figures, or percentages of the age groups, may be used.
The importance of such diagrams is that they can show, in pictorial form, the varying population structures of different types of society. For example, in many less developed countries the pyramid will have a very wide base; these are termed progressive pyramids because they suggest future population growth. In contrast, the regressive pyramids characteristic of Western nations, where there are fewer children and more old people, are more cylindrical; population is likely to decrease. In between the two are stationary pyramids which show a balance between old and young and no population growth.
Age-sex pyramids
