4 roots. "Music Guide" is a MOOC course provided by Shandong University. The musical instrument in the picture on page 38 has 4 strings. It is a violin, a stringed instrument with a total of four strings. The friction between the strings and the bow produces vibrations, which are then transmitted to the back plate through the sound columns in the sound box. The vibrations generated by these parts produce harmonious and bright piano sounds.