According to the modern qin making industry, at that time their adhesive strength was not enough, so they used iron nails to firmly connect them. But according to research, there were already physical objects using animal glue in Egypt 2700 years ago; Even 2500 years ago in China, there was a record in the "Kao Gong Ji" that "deer glue is green and white, horse glue is red and white, cow glue is fiery red, rat glue is black, fish glue bait, rhino glue is yellow". In modern times, it is also common to see examples of amateur qin makers buying fish or animal tissue to boil glue themselves because they cannot buy finished glue. It can be seen that animal glue is a very simple and reliable craft that ancient people (ancient, not modern people like the 17th century) had already mastered. So, in the golden age of violin in the 17th century, did Europeans at that time not master the technology of animal glue? I can't believe this. We know that the piano head is also a sound producing component, and my intuition tells me that this kind of hard connection with iron nails may have a positive impact on the sound of the piano, because the speed of sound transmission in metal and glue is definitely different. If it is only to reinforce or position the piano head (assuming they have no confidence in animal glue, but is it possible? The extraction technology of animal glue is so simple that modern amateur enthusiasts can easily extract high-quality glue by boiling it in a pot), do they use three iron nails? I think the purpose of using so many iron nails is more likely to be to optimize sound conductivity.

The mortise and tenon soft connection structure of modern qin heads (why is it called soft connection? Because its sound transmission does need to be filtered through a layer of glue)