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Music Theory First Aid for Struggling Students
This site provides music theory information on such topics as rhythm, intervals, scales, chords, etc. Also, there is a fine section which describes the basic concepts of form and structure.
OpenStax
Open Stax: Catherine Schmidt Jones: Melody
Learn about one of the basic elements in music, melody. Discover the ideas of melodic shapes and phrases as well as motifs and themes, and how to present these topics to children.