Music of the KOTO

     The music of the 13-string koto (琴, or 箏) zither (or sou) is called So-kyoku (筝曲, “Sou music”), or alternatively Koto-uta ("koto songs").  The koto first appeared in the ancient court Gagaku ensemble (wagon), but later went on to develop its own repertoire in the Edo period.

     In contrast to fretted string instruments, the koto's movable bridges are used to create different scale tunings (hira-joshi, kumoi-joshi) for each composition.  Some of the techniques associated with koto include opposite-the-bridge bending, string slides, and plucked harmonics.  Structurally, a typical koto piece may open with a brief  "overture" statement, followed by variations slowly building up to a climax, and then ending with a subdued coda.

Vocal and Instrumental Forms
     Koto pieces with vocals are called Kumiuta, and they are essentially suites of short poems (dan) with koto solo interludes.  Solo koto without voice is called Shirabemono (or Danmono) and these have a theme and variations/rondo structure (ie - with connecting interludes). The Jiuta form (地歌, 地唄, ぢうた, also used with shamisen ensembles) combines both vocal and instrumental sequences (described further below).

Koto Schools:

Tsukushi-goto:
     The earliest solo koto school (from 16th century Kyoto), Tsukushi-goto is a slow and languid style, and its "first" piece ("Fuki") was inspired by Gagaku's "Etenraku" and imayo forms (it uses Gagaku scales as well).  This style acts as something of a bridge between the ancient gagaku koto styles and "modern" koto.  Unfortunately this style is practically extinct.

Yatsuhashi-Ryu:
     Developed in the 17th Century with blind soloists in mind, the Yatsuhashi style took elements from Tsukushi-goto and applied them to folk styles and scales (such as the folk “in” scale).  Some pieces attributed to this school include “Rokudan” ("6 Sections/Variations), “Hachidan” ("8 Sections"), and “Midare” ("Chaos").  "Rokudan" (or "Rokudan no Shirabe") is the most famous koto work, and as is typical of the genre, opens with a 2-measure theme overture.  Another favorite, "Godan Ginuta (Kinuta)" is a koto duet with 5 variations ("Kinuta" is fabric pressing).  It’s 5th variation’s 2nd koto part is notable because it incorporates Rokudan’s 1st variation.

 

Rokudan no Shirabe (6 Variations)
1. 0:14
2. 2:38
3. 4:22
4. 5:24
5. 6:03
6. 6:48

  

Godan Kinuta (5 Variations Fabric Pressing Music) (live with backing tape)

 

Ikuta Ryu:
     This Kyoto school developed the concept of the koto chamber group in the form of the Jiuta trio (also called Sankyoku, 三曲).  In this ensemble, the koto is joined by a shamisen and either a kokyu (vertical bowed instrument) or a shakuhachi flute.  Fronted by a vocalist, this group uses tegotomono ("instrumental interlude") form, where vocal sections alternate with dazzling instrumental sections.  The trio interludes are usually thematically unrelated to one another and act only to prepare for the next song or poem.  This form somewhat combines techniques from both kumiuta and shirabemono.  Additionally, a jiuta trio may sometimes accompany Kyomai geisha dances from Kyoto.
 
     Vocally, the Jiuta form is typified by deep, expressive singing (jiuta means “earth/fundamental song”).  It employs sudden swooping melodic shapes in order to express suppressed emotions (atari).  Emphasizing precise technique, the koto plays the main melody, and the shamisen and shakuhachi loosely follow in heterophonic fashion.  In a piece using the Danawase compositional technique, a counter-melody is played on a second koto.  This has the effect of producing 2 arpeggios on the same chord (as opposed to a counterpoint line).  In similar Dangaeshi works, a second koto's melody from a different section (dan) is superimposed on the first koto's melody, creating a harmonious collage effect of sorts (a variation can be found in "Godan Kinuta", above).
   

Sankyoku: "Shōchikubai" (edited) with koto/voice, shamisen, shakuhachi

Yamada Ryu:
     This Tokyo school focuses on songs and voice (narrative singing), and originated in Western Japan.  A live broadcast can be seen here.

Modern Koto:
     Some of the more recent schools of koto were developed in the mid-20th century by composers such as Miyagi Michio (who invented the 17-string bass koto) and Nakanoshima Kinichi.  Contemporary kotos have as many as 30 strings (see Shin Miyashita and Aya Asakura).

Composition: "Tegoto" by Michio Miyagi

 
Composition: "Freeze" (凍る) by Shinichiro Ikebe

https://www.britannica.com/art/Japanese-music/Koto-music 
Koto Wiki 
Japanese Music & Musical Instruments, William Malm, 1959