Wojijing (sic!) 握奇經 "Classic of controlling by the unexpected [movements of the formations]" is a military treatise traditionally attributed to Feng Hou 風后, a minister of the mythological Yellow Emperor 黃帝. It is said that Jiang Shang 姜尚 (i.e. Qi Taigong 齊太公), a counsellor to King Wu of Zhou 周武王 (ca. 1050 BCE), extended the book. There is also an integral commentary embedded written by the Former Han-period 前漢 (206 BCE-8 CE) strategist Gongsun Hong 公孫弘 (200-121 BCE).
The book first appears in the imperial bibliography of the official dynastic history Songshi 宋史. Another hint for its late composition is the statement that it was written by Li Quan 李筌 xxx from the Tang period 唐 (618-907). The book is also known under the names Wojijing 握機經 "Classic of controlling through flexibility" or Wojijing 幄機經 "Classic of the flexibility of the [commander's] tent". It is only one short juan-long.
The Wojijing mainly deals with arrangement and movements of formations during battle. There are eight battle arrays (ba zhen 八陣), the regular (zheng 正) formations of Heaven, Earth, Wind and Clouds, and the flexible or "unexpected" (jī [sic!] 奇) formations of the Dragon, the Tiger, the Bird and the Snake. The commander-in-chief is placed in the middle of these formations from where he moves the platoons and companies. His own body of troops is called yuji 餘奇 "reserve unit". During the building up of the the arrays light cavalry troops (youjun 游軍) advance to keep a close watch on the movements of the enemy. The light cavalry is then drawn back to the rear. During battle, when the formations advance, the light cavalry again comes to the front to protect the own phalanxes and to harass those of the enemy.
About the arrangement of the eight arrays and their movement, virtually nothing is said in the main text. These points are explained in commentaries, as far as possible.
The earliest surviving prints are from the Jiguge Hall 汲古閣 and the Hongxing shangfang Studio 紅杏山房.