Most of the copybooks we see today are rubbings of inscriptions. Their calligraphy was written by a special calligrapher, and the person who carved the tablet was made by another craftsman, so there was a problem, that is, the distortion of calligraphy, which was inevitable in the process of carving the tablet.
Therefore, ancient calligraphers thought that if we learn calligraphy, the best way is to write ink calligraphy, rather than choose rubbings, because the inscriptions are definitely distorted in the process of seal cutting, and there is no problem of inscription distortion in ink calligraphy.
This approach may be somewhat abstract, so let's take the "Jiuchenggong Monument" in Ou Yangxun as an example. According to historical records, when Ou Yangxun wrote again, he first dipped his brush in cinnabar, then wrote it directly on the stone, and then. The craftsman chiseled directly according to the vermilion characters he wrote, which is the calligraphy characters we see today.
Of course, there is another way to carve a tablet, which is to write it on paper with a brush, and then. If we want to keep the original, we will draw the original with another piece of paper, then spread this drawn paper on the stone, and then try to draw an outline on the stone, and then we will ask the stonemason to chisel it again.
In this case, you will see that there are many places in the process of carving the monument, and there will be distortions. For example, after the calligrapher finishes writing, we will draw it with paper, which is a distorted process. Then we draw it on the stone, which is another twisting process, and then we ask craftsmen to chisel it, which is also a twisting process.
Therefore, if calligraphers want to keep their calligraphy as undistorted as possible, the fewer procedures they take, the better.
Another way to reduce distortion is to let craftsmen know some calligraphy. For example, seal engravers in the Tang Dynasty knew some calligraphy at that time, so that they could understand the calligrapher's intention of using a pen in the process of seal engraving. Then in the process of chiseling, we will try to avoid distortion.
Inscriptions are sometimes the only way for us to know the face of ancient calligraphers. For example, many of Ou Yangxun's calligraphy are ink calligraphy, but there are no ink calligraphy circulating in his regular script, only stone tablets, so we can only rely on stone tablets to understand his calligraphy.