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The trio is a section that usually follows immediately after a minuet in a minuet and trio (or, in later music, in a scherzo, march, and so forth). Therefore, the trio is the B section, rounding out the minuet and making it ternary form (denoted A-B-A, or A-B-A' if the repeat of the minuet is changed).
A trio forms the second section of a full Minuet and Trio. The trio usually contrasts sharply with the key used in the A section, although this is not by any means a requirement. The name trio comes from a very old style of composing the middle section for a trio; composer Lully (a French-Italian baroque composer), for example, commonly scored his trio sections for two oboes and a bassoon. According to the Minuet and Trio article on Course Notes' Music Theory section, “The ‘Trio’ is noticeably lighter and sweeter than the Minuet.” although this is unlikely to be universally true.
Course Notes' Online Music Theory Articles
William E. Caplin's book Classical Form: A Theory of Formal Functions... on Google Books