Zhang Xu

8th century), courtesy name Bogao (伯高), was a Chinese calligrapher and poet of the Tang dynasty.

Legend has it that whenever he was drunk, he would use his hair as brush to perform his art, and upon his waking up, he would be amazed by the quality of those works but failed to produce them again in his sober state.

There is an anecdote that says he grasped the essence of cursive writing by observing some porters fight for their way with the guard of honor of some princess, and by watching the solo performance of a famous sword-dancer named Lady Gongsun (公孫大娘).

[1] Under the excitement of art (and wine), he became oblivious of social expectations, and would often fling off his cap in the presence of princes and nobles.

The duo is affectionately referred to as "the crazy Zhang and the drunk Su" (顛張醉素).