While traditional kotos have 13 strings, the koto called a jūshichigen (lit. 'seventeen strings') was invented by Miyagi Michio in the Taisho era to play the role of a bass instrument in ensembles. In modern times, it has been widely adopted across all of the various koto schools, and quite a few solo pieces have been composed as well.
Not only has the number of strings increased, but the thickness of the strings and the overall size of the instrument have also increased. Early on, there were distinct 'large' and 'small' types of jūshichigen, but nowadays the most common ones are around 210cm in length.
Some also have an 18th string added, but these are still referred to as a jūshichigen. Note that when the word jūshichigen is used as a proper noun, the character for gen (string) should be 絃, but in this library we have used the simpler variant 弦 for consistency.