It's just a miniature version of a door hinge, something every household has.
Normal door hinges are large, with two grooves on the top and bottom, with a round wooden piece in between, which holds the wooden door; when fitted into the grooves, it allows the door to open and close smoothly.
However, as it's not very smooth and has high friction, it often makes a "creak, creak" noise when opened or closed.
This design, when scaled down, can be applied to pencil cases, and the noise is also significantly reduced.
Liu the carpenter was once again amazed by the design on the blueprint.
He had made countless doors for the villagers but had never considered that the hinges on doors could be downsized and applied in this way.
Of course, there are limitations to the environment; from ancient times to the present, the scholars' pen cases have always been sliding boxes, which are very convenient to use, so no changes were made.
