The growth status of the bones can be used as a maturation assessment system by comparing an individual’s state of development with those depicted in an atlas or standard—that is, a publication picturing skeletal development at many levels, each of which is assigned a skeletal age. The most common bones used for this purpose are the hand and wrist bones (figure 5.4). Thus, from an X ray of a person’s hand and wrist, we could determine a skeletal age by finding the picture in the atlas closest to the individual’s X ray. For example, a boy might have a skeletal age of 8.5 years because his hand and wrist X ray is most similar to the extent of ossification in the standard for 8.5 years. If his chronological age is under 8.5, we would know that he is an early maturer; if it is over 8.5, we would know that he is a late maturer. Skeletal age can easily be a year ahead of or behind chronological age, emphasizing how much variation is possible in maturation status even among those born on the same day.
This is an excerpt from Life Span Motor Development, Fifth Edition.