So far we have been concerned with body size and shape as exemplified by height, weight and their relationship. But growth is a movement through time, and progress may be fast or slow. Two children may reach an identical ultimate height, but one with a tempo of growth (to use Franz Boas’ phrase adopted from classical music) which is slow, another with a tempo which is rapid. One girl reaches menarche, the first menstrual period, at 11.0 years, another at 15.0 years. Within a particular population final adult height is not related to the speed with which it is reached; on average, early and late developers end up almost exactly the same in height. Even in shape there is only a small difference, late-maturers ending usually as more linear people with a lower weight-for-height.

Thus tempo of physical development has to be studied separately from body size; there are differences in tempo between populations and no a priori reason why these differences should be related to differences in height and weight. Rate of maturation in body size may be measured by the percentage of mature height reached at successive ages, but this is only known for any individual after his whole growth has been completed. More widely used measures of maturity, therefore, are skeletal maturity (or bone age), dental maturity and pubertal maturity (or secondary sex character age). These measures are not dependent on childhood or adult body size. Each measure reaches the same final point in all normal persons (unlike height and weight).