Why does the guzheng have 2 1 strings?

Hello, the traditional guzheng is generally thirteen strings. Later, after improvement, there were 21 strings and 25 strings, but now the most popular one in China is 21 strings. At present, the Japanese guzheng is still thirteen strings. Guzheng is a representative national musical instrument in China, so there are only 1, 2, 3, 5, 6 notes in the 18th degree, which is a pentatonic scale.

4 and 7 can be pressed on 3 and 6.