This article is from the Drum corps FAQ, by Cathy Doser cathyd@halcyon.com with numerous contributions by others.
The number of members allowing depends on the Class that the corps
is competing in. The Open Class corps may field up to 128 performers
(including the drum majors). This number comes from the fact that
when the rule was devised, this was the average number of people that
could fit on three buses. The idea was to minimize the travelling
costs for the corps. Division II corps may field up to 90 people and
Division III corps may field up to 60 members.
These are the formal differences between the classes of drum
corps, but in reality the Open Class corps tend to be the ones that
attract the better instructors, go on longer tours, and generally
perform better, while the Division II and III corps tend to be more
oriented towards being a youth activity. Note that these are
generalities so that any particular corps could be anywhere in the
spectrum of youth activity or innovators in performance art.
 
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