The traditional koto has 13 strings. Moveable ivory bridges, called ji, are placed under each string, and moving them up or down the length of the koto raises or lowers the pitch. There are about 15 set positions for the ji, known as choshi, and they determine the overall tuning of the instrument. All traditional koto music uses one or more of these choshi.

The number of strings has remained constant since the koto was imported from China as part of the gagaku Imperial court orchestra during the Nara Period (eighth century). In the 20th century, however, hogaku musicians began experimenting with instrumental innovations and modern playing techniques.

One obvious innovation was to add more strings to the koto in search of a greater pitch range and more flexibility in the tuning.

The early 20th century was a fertile time for experimentation in hogaku and attempts to expand its boundaries. Some were quite successful; some never got off the ground.

One of the first such attempts was by the well-known koto player and composer Michio Miyagi, whose 17-string koto (invented in 1921) became Japan's first true bass instrument. Miyagi composed a number of works using it, and they are still performed widely today.

Perhaps buoyed by the success of the 17-string koto, in 1929 Miyagi came up with an 80-string koto, aiming to put it on a par with the piano and allow a full range of chromatic pitches. Miyagi played it in concert in the fall of 1929, but the instrument proved unsuccessful. It was never played again and was ultimately destroyed in the fire bombings of the war (although a replica was made for the opening of the Miyagi Michio Museum in 1978).

Keiko Nosaka, a gifted and prolific koto performer, developed the 20-string koto in the late '60s and the 25-string koto even more recently. The range of these kotos is about the same as the 13-string koto, but the extra strings allow for greater flexibility in tuning. Both the 20- and 25-string koto are popular and often used in contemporary hogaku compositions.

Less known is the 30-string koto, developed by the koto performer and composer Shuretsu Miyashita. Like Miyagi, Miyashita wanted to expand the koto's range, but he also wanted to keep the basic koto shape and sound unchanged. Miyashita first came up with the idea of the 30-string koto around the time of Miyagi's experimentations (Miyashita had been a student at the school for the blind where Miyagi taught), but there were problems finding the right kind of strings. After the war and the invention of various nylon fibers, Miyashita found the right gauge of string for the upper notes, and finally had a 30-string koto produced in 1950.

The 30-string koto essentially has the bass range of a 17-string koto and the refined tonal colors of the 13-string koto. It is versatile, having a range of four and a half octaves. Like Miyagi, Miyashita also composed a large number of pieces especially for the instrument he invented.

Although the 30-string koto never attained the popularity of the 17- or 20-stringed instruments, it is still used by Miyashita's descendents and students. When Shuretsu Miyashita passed away in 1993, his eldest daughter took over the name and artistic mantle. She will present a recital of works by her father and other contemporary hogaku composers later this month.

Koto Sanjugen Recital, 1 p.m. Dec. 24, National Theater Small Hall. Admission 4,000 yen in advance, 4,500 yen at the door. For information and tickets call the Shuretsu Sha main office, (03) 3910-3548. After the recital, beginning at 2:30, there will be another recital featuring Miyashita's students. The tickets include both concerts.

The other concerts I recommend for the month of December are all of classical music and all at the Kioi Small Hall, designed especially for hogaku concerts.

Three jiuta concerts will be presented by three very different masters of the art. Jiuta, "local songs" developed in Edo Period Osaka and Kyoto, are accompanied by shamisen, koto and shakuhachi. Jiuta instrumental accompaniment is often flashy, technically demanding and very entertaining.

Norie Tomobuchi will offer both solo and accompanied jiuta in her concert, also featuring the shakuhachi living national treasure Reibo Aoki.

"Tomobuchi Norie: Nihon no Uta" 7 p.m. Dec. 7 at Kioi Small Hall, between Yotsuya Station and Akasaka Mitsuke subway station. Admission 4,000 yen. For information and reservations call Kioi Hall ticket center, (03) 3237-0061 or see the Kioi Hall Web site at www.kioi-hall.or.jp

Kunie Fujii is an undisputed jiuta master. Her sonorous voice and rich shamisen tone echo her long years of experience. She will perform a short solo piece, "Sode no Tsuyu," the complaint of a woman who waits the long autumn evening for a lover who never shows, and the lengthy, lively "Mitsuyama," taken from the noh play of the same name.

Although the concert has only two pieces, each represents the ultimate refinement of style: one simple, short and to the point, and the other lengthy and full of intriguing instrumental digressions and complicated lyrics.

Fujii Kunie: Jiuta no Kai, 7 p.m. Dec. 8 at Kioi Small Hall. Admission 4,000 yen. For information or reservations call Mizuno Music Office, (045) 961-8250.

Tomoko Sunazaki is a respected musician of the Michio Miyagi tradition, mentioned above. Her career has mostly been devoted to contemporary hogaku, but her Dec. 13 recital will, for a change, feature only classical pieces. Accompanying her will be shakuhachi great Hozan Yamamoto and koto players Shoin Yamase and Hiroe Yonekawa. She will present a program which is easy to understand and appreciate, ranging from koto classics as "Midare," to the animated jiuta piece "Azuma Jishi."