Sankyoku (instrumental trio) (三曲)

Sankyoku is the collective name for the three musical instruments: jiuta shamisen (shamisen, or sangen--a three-stringed Japanese banjo, for jiuta, or traditional songs with shamisen accompaniment), so (a long Japanese zither with thirteen strings), and kokyu (Chinese fiddle). Sankyoku is also the collective name for the types of music played on each instrument: jiuta, so-kyoku (so music), and kokyu-gaku (kokyu music). Shakuhachi (bamboo flute) was added to sankyoku later. Sankyoku can also indicate the sankyoku gasso (sankyoku ensemble).

Origin of sankyoku
It is not clear when the word sankyoku was first used, but there have been some instances of such use in a sense that the three instruments were played together. Jiuta shamisen, so and kokyu had originally been the musical instruments played by blind musicians in the Todo-za (traditional guild for the blind) since the early Edo period. They had sequentially produced the music of jiuta, so-kyoku, and kokyu-gaku played on each instrument respectively. It is assumed that the word 'sankyoku' started to be used to distinguish these instruments and music genres from 'hei-kyoku' (music played on heike-biwa, or Japanese lute used as accompaniment for the recitation of Heike Monogatari), their area of expertise, which had been performed for a long time.
However, some records from the mid Edo period indicate that the combination of so, kokyu, and shakuhachi was called 'sankyoku.'
This could mean that any combination of three instruments were simply called 'sankyoku' (in fact, the Todo-za members did not play shakuhachi). The word 'sankyoku' appeared in literature from around this time.
Although various music instruments were played in concert in the early Edo period, there is no evidence to show that any combination of music instruments was called 'sankyoku.'
As they were being established as art music, jiuta, so-kyoku and kokyu-gaku each came to retain their own musical composition; each of them became established as a different genre of music. The music instruments used for them started to be played in concert centering around jiuta from around the mid Edo period. To play all three instruments of sankyoku in concert, in particular, is called sankyoku awase or sankyoku gasso.
It is not clear which came first, 'sankyoku' or 'sankyoku gasso.'

Shakuhachi was not an instrument played by the Todo-za members, but it had close ties to kokyu from old days. In addition, shakuhachi came to be played in concert often with the shamisen and so from around the mid Edo period, and it fully joined them after the Meiji period. The sankyoku gasso, accordingly, has had two types of ensemble in general: ensemble of shamisen, so and kokyu, and that of shamisen, so and shakuhachi. However, the latter is far more common today. Some sankyoku musicians do not even include kokyu in 'sankyoku' and ignore it because the number of musicians who can play kokyu has declined since the Meiji period. It is inappropriate, however, to exclude kokyu from sankyoku and to regard it as a thing of the past because kokyu was originally one of the instruments of sankyoku, kokyu-gaku is still passed down, and the sankyoku gasso with kokyu is still played.

Musicians who play shamisen, so, or kokyu often play other instruments, but shakuhachi players usually play only shakuhachi. Sankyoku gasso has been often portrayed in nishikie (colored woodblock print) probably because the visual of sankyoku gasso looks stunning. These nishikie mostly include kokyu.

Since the mid Edo period, more and more, jiuta, so-kyoku, and kokyu-gaku tended to share the same songs for ensemble while each maintaining their own songs. They gradually became united and had become inseparable by the end of Edo period. Shakuhachi was also added to the ensemble from the Meiji period, and jiuta and so-kyoku were more and more played on shakuhachi as its repertoire. Music composed for shamisen today is played in concert with jiuta shamisen and so; overall, the music played on these instruments are unified while each retaining its own style. Therefore, four types of music, jiuta, so-kyoku, kokyu-gaku, and shakuhachi-gaku, are collectively called 'sankyoku', and the word 'sankyoku' has been used to distinguish itself from other traditional Japanese music such as nagauta (long epic song with shamisen accompaniment), gidayu-bushi (musical narrative with shamisen accompaniment), kiyomoto-bushi (musical narrative with shamisen accompaniment), biwa-gaku (biwa music), and nogaku (the art of Noh). Today's sankyoku musicians form music associations such as 'Sankyoku-kai' (Sankyoku Society), 'Sankyoku-renmei' (Sankyoku Federation) and 'Sankyoku-kyokai' (Sankyoku Association) in addition to their own schools.

Sankyoku (the three instruments) used for other than sankyoku (sankyoku music)
The three instruments of 'sankyoku' are sometimes used for other schools of shamisen music perhaps because jiuta has a long history as shamisen music and has been the norm for other schools of shamisen music. For example, among the gidayu-bushi, accompaniment for ningyo joruri (traditional Japanese puppet theater) and kabuki (traditional Japanese drama), 'Akoya-no-kotozeme' is the most famous, and the sankyoku of jiuta shamisen, so and kokyu are all played in it. Also in the nagauta music of "Sankyoku Ito no Shirabe" and "Sankyoku Sho-chiku-bai," things about shamisen, so and kokyu are chanted and the music of each instrument is played only on shamisen (without actually using the instruments of sankyoku).

Notes
Among so-kyoku, the Tsukushi-goto (a school of so-kyoku) and the Yatsuhashi school do not play in concert with other instruments since they retain the music of the beginning of the Edo period and the early Edo period almost in tact even today.

In the so music of the Yamada school, chuzao shamisen (shamisen smaller than jiuta shamisen) for joruri instead of jiuta shamisen is generally used for ensemble.

In the so music of the Yamada school, some pieces are played with (played in turn, i.e., exchange of performances) the music of joruri such as the Icchubushi (a school of joruri), a different genre from sankyoku.

Shamisen music other than jiuta is not included in sankyoku, nor played in the sankyoku gasso.

Shamisen, shakuhachi, and kokyu used in minyo (a traditional folk song) and folk arts are not included in sankyoku.