Đàn Tranh has a long soundbox with the steel strings, movable bridges and tuning pegs positioned on its top.... Đàn Tranh can be used either as a solo instrument, as part of various instrumental ensembles or to accompany vocal performances, the number of strings of Đàn Tranh increased to fifteen and the instrument was called Thập Ngũ Huyền Cầm. The standard version of Đàn Tranh, or the đàn Thập Lục had 16 strings and had been used between the nineteenth century and the late 1980s. In the late 1950s, South Vietnamese master musician and instrumental designer Nguyễn Vĩnh Bảo (b. 1918) began to design and construct instruments with 17, 19 and 21 strings. By the late 1980s, the 17-stringed Đàn Tranh has become the standard version of the instrument used throughout Vietnam. Larger instruments with 22, 24 and 25 strings have also been made in the 1980s and 1990s. https://en.wikipedia.org/wiki/dan_tranh