(Translated by https://www.hiragana.jp/)
Music Theory - Wikibooks, open books for an open world

This book discusses tonal music theory, specifically of the common practice period onwards, including jazz, blues, rock, and other modern styles. It focuses mostly on Western (i.e., Western European, Euro-American, and Afro-American) styles, however, all styles of music are discussed.

Basics

Common Practice Styles

Harmony

Tones, overtones, and harmonics

Beyond the Basics

Modern Styles

Styles in the Western Classical Tradition

Traditional and Folk styles

Composing

Further reading

Appendices