What are the artistic features of Dong Qichang's calligraphy?

Dong Qichang's artistic creation of calligraphy entered a free stage after he was 60 years old. In his later years, his representative works include Preface to Liu Gongquan's Lanting Collection, Imperial edict of III, Friends of Stone Stage, Yan Zhenqing Seating, etc. During this period, his calligraphy skills became more and more proficient, and he was able to integrate the artistic characteristics of various schools and make it more and more clear. He wants to get rid of the traditional old methods and create his own style. His works in this period, on the one hand, show the simplicity of calligraphy with seemingly casual and relaxed brushwork, on the other hand, reflect Gu Zhuo with poor brushwork.