The so-called "Yan Gu" means that the calligraphy features of Yan Zhenqing and Liu Gongquan (Yan and Liu) are vigorous, vigorous and as thin as bones.
And these two calligraphers, one is a great master in the prosperous Tang Dynasty (Yan Zhenqing) and the other is a calligrapher in the late Tang Dynasty (Liu Gongquan), how can they talk about the Sui Dynasty? When the Sui Dynasty was in a state of war and chaos, the people were in poverty. How could they have leisure to practice calligraphy and specialize in calligraphy?