I can clearly answer: it is written with a curved pen, which is specially used to write hard-pen calligraphy, and the inclination angle of the article can greatly change the line thickness. This kind of pen was very popular more than ten years ago, but it is less now, but it is still sold in the market. I was obsessed with writing this kind of characters more than ten years ago, and later I found it better to practice ordinary pens or brushes. Hope to adopt.