The book Tang Taizong Li Weigong wendui 唐太宗李衛公問對 "Li Weigong answering the questions of Emperor Tang Taizong", short Li Weigong wendui 李衛公問對, is a military treatise composed as a question-and-anwer dialogue between Emperor Tang Taizong 唐太宗 (r. 626-649) and Li Jing 李靖, Duke of Wei. It belongs to the Seven Military Classics (Wujing qishu 武經七書). The date of composition is not clear, it is traditionally attributed to the Song period 宋 (960-1279) writer Ruan Yi 阮逸, but this is far from certain. The Yuan period 元 (1279-1368) scholar Ma Duanlin 馬端臨 argued that the received version must have been edited during the reign of Emperor Shenzong 宋神宗 (r. 1067-1085).
The discussion between the ruler and his minister center around several antagonistic terms by which military tactics can be explained, namely unorthodox/orthodox (qi zheng 奇正), appearance/truth (xu shi 虛實), subject/object (zhu ke 主客), and attack/defense (gong shou 攻守). The book provides a lot of examples from practical warfare, like the deployment of troops and their lining-up during battle, the military system and strategic thought. It is shown that the orthodox formation has always to be adapted to the strength and the acitivity of the enemy, as well as to the territory and geographical conditions. As a very good example in the book how both types, the orthodox as well as the unorthodox tactics, was employed, is the battle of Huoyi 霍邑 in 617 when general Li Yuan 李淵, the eventual founder of the Tang dynasty 唐 (618-907), defeated Song Laosheng 宋老生, general of the Sui dynasty 隋 (581-618). With the fixed lines, the orthodox tactics or the real tactics, the enemy can be deceived while the unorthodox or void units attack him. This is the so-called demonstrating of a shape (shi xing 示形) which only served to conceal the real attackers. Territory can be used in a right way to make oneself subject or master of the enemy and to press the enemy into the position of the object. Attack and defense, Li Jing says, are interconnected with each other. Attack is the favourable turn of defense, and defense is the strategic plan or base for attack. During attack it is always necessary to hit the heart of the inimical troops and to shatter its fighting spirit. During defense it is vital to built up the defense walls and to stabilize the lines of the formation to strengthen the martial temper of the troops. Li Jing describes the importance of the so-called six-flower formation (liuhuazhen 六花陣) for large armies and camps which makes it possible that stronger units are able to protect the weaker ones.
The book has been studied for centuries and is still very valued, although it also contains some superstitious aspects like the dependance of cosmological forces like Yin and Yang 陰陽 and the belief in fortune-telling.