A population pyramid represents the proportions of different age groups in a given population as two bar graphs arranged side by side; conventionally, the left bar graph represents males, and the right one represents females. Each bar's length represents the proportion of the population that falls within a demographic.
Since ages in a population pyramid are represented ranging from youngest at the bottom to oldest at the top, the two graphs aligned side by side typically give the appearance of a pyramid. This is because fewer people in a population live into old age. The BBC notes, however, that it is possible to distinguish different qualities in a population through variations from the standard pyramid shape. For example, an aging population with a low birth rate would look like an upside-down pyramid.