Why doesn't the qin maker draw a horizontal line with a pencil directly from the F-hole cut on both sides of the panel before putting it together, and then use millimeters or two millimeters as a horizontal ruler on top, and then use the projection of the piano code foot as a basis to make vertical coordinates. Similarly, mark the unit mark, make grid marks within the experience range, and make ruler and grid marks at the corresponding position of the backboard, and the coordinates correspond to the mirror image of the panel. In this way, when calibrating the sound column, dental mirrors or tail holes, or video peeping probes can be used to accurately locate and ensure the vertical installation of the sound column, making it easy to adjust the reference. The traditional approach is to rely on experience and visual inspection, but visual inspection is actually the least reliable.

For example, if the upper part of the sound column is located at 2 and 4 of the XY coordinates of the panel grid, then the position of the back panel sound column should also be at 2 and 4 to ensure a vertical relationship. And because the internal coordinates correspond to the surface, it is easy to accurately determine the position relationship between the sound column and the piano code. Of course, the method of using incision paper is also good, but there are errors and uncertainties caused by the slope.