A biwa with four strings and four frets popularized during the Edo Period in Satsuma Province (present day Kagoshima) by Shimazu Nisshinsai. Modern biwas used for contemporary compositions often have five or more frets, and some have a doubled fourth string. The frets of the Satsuma biwa are raised 4 centimeters from the neck allowing notes to be bent several steps higher, each one producing the instrument's characteristic sawari, or buzzing drone. Its boxwood plectrum is much wider than others, often reaching widths of 25 centimeters or more. Its size and construction influences the sound of the instrument as the curved body is often struck percussively with the plectrum during play. The satsuma-biwa is traditionally made from Japanesemulberry, although other hard woods such as Japanese zelkova are sometimes used in its construction. Due to the slow growth of the Japanese mulberry, the wood must be taken from at least a 120 year old tree and dried for ten years before construction can begin. The strings are made of wound silk. Its tuning is A, E, A, B, for traditional biwa, G, G, c, g, or G, G, d, g for contemporary compositions, among other tunings, but these are only examples as the instrument is tuned to match the key of the player's voice. The first and second strings are generally tuned to the same note, with the 4th (or doubled 4th) string is tuned one octave higher. The most eminent 20th centurysatsuma-biwa performer was Tsuruta Kinshi, who developed her own version of the instrument, which she called the tsuruta-biwa. This biwa often has five strings (although it is essentially a 4-string instrument as the 5th string is a doubled 4th that are always played together) and five or more frets, and the construction of the tuning head and frets vary slightly. Ueda Junko and Tanaka Yukio, two of Tsuruta Kinshi's students, continue the tradition of the modern Satsuma biwa. Carlo Forlivesi's compositions Boethius and Nuove Musiche per Biwa had both been written for performance on the Satsuma model of the biwa designed by Kinshi Tsuruta and Yukio Tanaka. "These works presents a radical departure from the compositional languages usually employed for such an instrument. Also, thanks to the possibility of relying on a level of virtuosity never before attempted in this specific repertory, the composer has sought the renewal of the acoustic and æsthetic profile of the biwa, bringing out the huge potential in the sound material: attacks and resonance, tempo (conceived not only in the chronometrical but also deliberately empathetical sense), chords, balance and dialogue (with the occasional use of two biwas in Nuove Musiche per Biwa), dynamics and colour.
excerpt from wikipedia https://en.wikipedia.org/wiki/Biwa