17

This number is a prime.

The 17-stringed bass koto (or jūshichi-gen, literally "seventeen strings") is used in the album PRIME NUMBERS by Neptune/Watanabe. As a matter of fact, all the traditional Japanese instruments played in it contain prime numbers. The other instruments used are the 3-stringed shamisen, the 5-holed shakuhachi (bamboo flute), and the 13-stringed koto.