A musical composition is constructed as solidly as a building, and its essential materials are Rhythm, Harmony and Melody.
Rhythm lays the foundation, providing the listener with a skeleton shape of the work.
Click the link to listen to the song “America” from West Side Story. Notice that the beat is not steady but fluctuates between 2 groups of 3, and 3 groups of 2.
Melody is the changing of pitches that are interwoven with rhythm to provide a meaningful pattern.
Harmony provides the background and context of the work.
-- The Arts-World Themes, Geraldine Nagel