How do the yo and in scales shape koto music?

The yo and in scales serve as the fundamental tonal systems for koto music from the 17th century onward, shaping the instrument's diverse tuning practices. While the ancient court koto (gaku-so) derived its tunings from earlier ryo and ritsu modes, the Edo period saw the emergence of these two indigenous tonal systems.1

The yo and in scales categorize the various standard tunings used for the koto's 13 strings. These scales provide the structural framework for the instrument's repertoire, influencing how performers utilize the strings and manipulate pitch. For example, the hira-joshi tuning, which is central to foundational works like Rokudan, is representative of these evolving tonal concepts.12

Essentially, the yo-in system acts as the primary tonal source for Japanese music developed during and after the Tokugawa period, distinguishing it from the older courtly traditions and later Western-influenced compositions. By providing these pentatonic-based foundations, the scales dictate the melodic character and harmonic possibilities inherent in koto performance practice.12