This is the top level category for small associations of stars that are within close line-of-sight proximity to each other, and are smaller than open clusters. If it is uncertain whether two optical companion stars are gravitationally bound, they should be placed under the category Double stars. Triple stars should be placed under the subcategory Triple stars. If a group of four or more stars are physically close and gravitationally bound, it is a physical multiple star and should be placed under the subcategory Multiple star systems. Multiple stars should be categorized by their number of components.

Many stars have optical companions, and there is no clear-cut criterion for what constitutes a multiple star. It is suggested that editors determine whether or not stars should be added to this category by consulting the Washington Double Star Catalog or a similar catalog of double and multiple stars.