The age structure of a forest is often expressed as the area of the forest consisting mainly of trees of the same age (i.e., by age class) or by proportions of trees in different age classes. The age-class division used is dependent on the ages of trees of different species. For forest accounting purposes, it is useful to have the age-structure expressed in terms of timber volumes. This may be derived by combining data on the number of trees with the average volume of timber for a tree of a given age. Since the volume of a mature tree is much larger than for a young tree, the distribution of timber volume may be very different from the distribution of number of trees.