2. Explanation: It refers to the rope tied to the back of the bow, which can send arrows, and the line that makes sounds on musical instruments. In ancient times, it also refers to the hypotenuse of an isosceles right triangle.

3. Source: Xu Shen's "Shuowen Jiezi" string, bowstring also. Seen from the bow, it looks like a silk. (translation: strings, strings tied at both ends of the crossbow. The glyph takes the "bow" as the edge, like a string tied at both ends of the bow. )

4. Word combination: chord, string song, upper string, lower string, sine, fixed string, cosine, silk string, second string and orchestra.