The koto is the ancient Japanese string instrument with 13 strings stretched over movable bridges.
The koto is a long zither traditionally made from paulownia wood. Its strings run across the soundboard, and each string rests on an adjustable bridge. Moving the bridges changes the pitch, allowing the player to set the instrument for different tunings.
The instrument entered Japan from China during the Nara period, when it was used in court music. Over time, Japanese performers developed local schools and repertoires, including the influential Ikuta-ryū and Yamada-ryū traditions. Players normally pluck the strings with picks worn on the thumb, index finger, and middle finger.
The standard modern koto has 13 strings, although larger contemporary instruments with 17 or more strings also exist. That variation explains why a general statement that every koto has exactly 13 strings can be misleading. The Japanese shamisen is a different instrument: it is a fretless, three-stringed lute played with a large plectrum.