A system of three or more stars bound by their mutual gravitational attraction. It is estimated that about one-third of all known binaries are actually triple. The proportions decrease with greater multiplicity. Systems with six components are known, but they are rare (less than 1 % of all multiples). In triple systems there is often one relatively close pair, with the third component in a wider orbit. Quadruple systems, however, occur approximately equally in 2 : 2 (2 pairs) and 1 : 1 : 2 hierarchies.