A classification of open clusters according to three criteria: degree of central concentration (from I, the most concentrated, to IV, little different from the surrounding stars); range of brightness of individual stars (from 1, small range, to 3, large range); and the total number of stars in the cluster (p meaning poor, less than 50 stars; m, moderately rich, 50–100 stars; r, rich, more than 100 stars). The suffix ‘n’ indicates that nebulosity is involved with the cluster, as in the Pleiades which is classified as I 3 r n. The system was introduced by the Swiss-born American astronomer Robert Julius Trumpler (1886–1956).