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A system for classifying the visual appearance of open clusters of stars, published by Robert J. Trumpler in 1930. The system describes the degree of central concentration of the cluser (Roman numerals I to IV in order of decreasing concentration and contrast with the background star field), the range of brightness covered by member stars (1 to 3 in order of increasing brightness range), and the apparent number of members (or "richness") using the codes p ("poor") for fewer than 50 stars, m ("moderately rich") for 50-100 stars, and r ("rich") for more than 100 members.
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