What are round pens and astringent pens?

Calligraphy pays attention to "roundness" and "astringency" with pens. The round pen is the pen used in the center, so the refill can often be used when stippling. Such lines are full, three-dimensional and three-dimensional, which can show substantial strength. Sewing a pen is to artificially create contrarian lines, so that the written lines are not slippery, but calm, dignified and vigorous, like withered vines.