What kind of calligraphy is Preface to Lanting Collection;

The running script Preface to Lanting Collection is also called Preface to Lanting Collection, Preface to Lanting Banquet Collection, Preface to He Lin, Preface to Zan and Zan Tie. It was written by Wang Xizhi, a calligrapher in Jin Dynasty. His calligraphy is of great artistic value, and it is called the three major calligraphy posts together with Yan Zhenqing's "Ji Ming's Nephew's Draft" and Su Shi's "Cold Food Post".