Twelvefold way

https://en.wikipedia.org/wiki/Twelvefold_way https://en.wikipedia.org/wiki/Enumerative_combinatorics

In combinatorics, the twelvefold way is a systematic classification of 12 related enumerative problems concerning two finite sets, which include the classical problems of counting permutations, combinations, multisets, and partitions either of a set or of a number. The idea of the classification is credited to Gian-Carlo Rota, and the name was suggested by Joel Spencer.