The Shuduan 書斷 "Judgments on calligraphies" is a book on calligraphy written by the Tang period 唐 (618-907) scholar Zhang Huaiguan 張懷瓘. He came from Hailing 海陵 (modern Taizhou 泰州, Jiangsu) and was an assistant (gongfeng 供奉) in the Hanlin Academy 翰林院. He was famous for his mastering the styles of the standard script (zhenshu 真書), the cursive "running script" (xingshu 行書), and the super-cursive "grass script" (caoshu 草書), and his expertise in the small seal script (xiaozhuan 小篆) and the broad chancery style (bafenshu 八分書). Of his writings, only the 3 juan "scrolls" long Shuduan has survived, as well as the treatises Shuyi 書議, Shugu 書估, Pingshu yaoshi lun 評書藥石論, Guwen dazhuan shuzu 古文大篆書祖, Liutilun 六體論 and Yongbi shifa 用筆十法.
The book Shuduan is an analysis of the works of famous ancient masters in calligraphy, like Gu Kaizhi 顧愷之, Lu Zhenwei 陸探微 or Zhen Sengyao 張僧繇. In the first juan ten various styles of script are explained, from the ancient script (guwen 古文) and the large seal script (dazhuan 大篆 and zhouwen 籀文) to the small seal script, the normal chancery script (lishu 隸書), the bafen style, the regular grass script (zhangcao 章草), the "flying white" style (feibai 飛白), and the genuine grass script. Zhang Huaiguan describes the origin, history and use of these styles and assesses its meaning in the field of calligraphy. The second and third juan provide ratings of calligraphers and their work. They are ranked according to three categories, namely "divine" (shen 神), "extraordinary" (miao 妙) and "professional" (neng 能). The first and best category includes 25 works, the second category 98 creations, and the third category 107 oeuvres. There are 68 persons included, plus 38 in an appendix, to which Zhang Huaiguan also provides short yet concise biographies. These are important because most of the ancient calligraphers are not to be found among the biographies in the dynastic histories.
The Shuduan is included in Zhang Yanyuan's 張彥遠 book Fashu yaolu 法書要錄, and in the collectanea Baichuan xuehai 百川學海, Gezhi congshu 格致叢書, Shuofu 說郛 (Wanwei shantang 宛委山堂 edition) and Siku quanshu
Source: Li Xueqin 李學勤, Lü Wenyu 呂文鬰 (1996). Siku da cidian 四庫大辭典, vol. 2, p. 1796. Changchun: Jilin daxue chubanshe.