Containment hierarchy

A containment hierarchy is a collection of strictly nested sets. Each entry in the hierarchy designates a set such that the previous entry is a strict superset, and the next entry is a strict subset. For example, all rectangles are quadrilaterals, but not all quadrilaterals are rectangles, and all squares are rectangles, but not all rectangles are squares. (See also: Taxonomy.)

• In geometry: shape, polygon, quadrilateral, rectangle, square
• In biology: animal, bird, raptor, eagle, golden eagle
• The Chomsky hierarchy in formal languages: recursively enumerable, context-sensitive, context-free, regular
• In physics: particle, elementary particle, fermion, lepton, electron