Dunbar's number is a theoretical cognitive limit to the number of people with whom one can maintain stable social relationships. These are relationships in which an individual knows who each person is, and how each person relates to every other person. Proponents assert that numbers larger than this generally require more restricted rules, laws, and enforced norms to maintain a stable, cohesive group. No precise value has been proposed for Dunbar's number, but a commonly cited approximation is 150. Dunbar's number was first proposed by British anthropologist Robin Dunbar, who theorized that "this limit is a direct function of relative neocortex size, and that this in turn limits group size … the limit imposed by neocortical processing capacity is simply on the number of individuals with whom a stable inter-personal relationship can be maintained." On the periphery, the number also includes past colleagues such as high school friends with whom a person would want to reacquaint the
Tyree Glenn, who had the unusual double of trombone and vibes, was an important asset at various times to both Duke Ellington and Louis Armstrong. Glenn became a longtime member of the Cab Calloway Orchestra (1939-1946). Glenn visited Europe with Don Redman's big band (1946). During his association with Ellington (1947-1951), he was an effective wah-wah trombonist in the Tricky Sam Nanton tradition and Ellington's only vibraphonist, being well-featured on the "Liberian Suite." During the 1950s, Glenn worked in the studios, led his quartet at the Embers, and freelanced in swing and Dixieland settings. Other than some European dates in 1947, Glenn's only extensive opportunity to record was for Roulette (1957-1958 and 1961-1962). During 1965-1968, he toured the world with Louis Armstrong's All-Stars. After leaving Armstrong, Tyree Glenn led his own group during his last few years.