The term somatotype is used in the system of classification of human physical types developed by U.S. psychologist W.H. Sheldon. In Sheldon’s system, human beings can be classified as to body build in terms of three extreme body types:

1. endomorphic, or round, fat type;
2. mesomorphic, or muscular type; and
3. ectomorphic, or slim, linear type.

A somatotype number of three digits is determined for an individual classified by the system, with the first digit referring to endomorphy, the second to mesomorphy, and the third to ectomorphy; each digit is on a scale of 1 to 7.

1. the extreme endomorph has the somatotype 711,
2. the extreme mesomorph 171, and
3. the extreme ectomorph 117.

The classification numbers are negatively correlated, so that a high number in one class precludes high numbers in the others; in practice, extreme types (711, 171, 117) are rare or nonexistent, and the person of normal build has a somatotype approaching 444, evenly balanced between extremes.

Ectomorph, a human physical type  tending toward linearity, as determined by the physique-classification system developed by the American psychologist W.H. Sheldon. Although classification by the Sheldon system is not absolute, a person is classed as an ectomorph if ectomorphy predominates over endomorphy and mesomorphy in his body build. The extreme ectomorph has a thin face with high forehead and receding chin; narrow chest and abdomen; a narrow heart; rather long, thin arms and legs; little body fat and little muscle; but a large skin surface and a large nervous system. If well fed, he does not gain weight easily; if he becomes fat, he is still considered an ectomorph, only overweight.

Endomorph, a human physical type  tending toward roundness, as determined by the physique-classification system developed by American psychologist W.H. Sheldon. The extreme endomorph has a body as nearly globular as humanly possible; he has a round head, a large, round abdomen, large internal organs relative to his size, rather short arms and legs with fat upper arms and thighs, but slender wrists and ankles. Under normal conditions the endormorphic individual has a great deal of body fat, but he is not simply a fat person; if starved, he remains an endomorph, only thinner.

Mesomorph, a human physical type that is marked by greater than average muscular development, as determined by the physique-classification system developed by American psychologist W.H. Sheldon. Although the Sheldon system of classification does not make absolute distinctions between types, a person is classed as a mesomorph if mesomorphy predominates over endomorphy and ectomorphy in his body build. The extreme mesomorph has a square, massive head; broad, muscular chest and shoulders; a large heart; heavily muscled arms and legs; and minimal body fat. He tends to develop muscle easily. His muscular development can usually be distinguished from that of one who has developed his muscles through body-building exercises.