The concept of continuous and concerted rotating fire, the countermarch, may have been implemented using crossbows as early as the Han dynasty, but it was not until the Tang dynasty that illustrations of the countermarch appeared. The 759 CE text, "Tai bai yin jing" (太白陰經) by Tang military official Li Quan (李筌), contains the oldest known depiction and description of the volley fire technique. The illustration shows a rectangular crossbow formation with each circle representing one man. In the front is a line labeled "shooting crossbows" (發弩) and behind that line are rows of crossbowmen, two facing right and two facing left, and they are labeled "loading crossbows" (張弩). The commander (大將軍) is situated in the middle of the formation and to his right and left are vertical rows of drummers (鼓) who coordinate the firing and reloading procedure in procession: who loaded their weapons, stepped forward to the outer ranks, shot, and then retired to reload. According to Li Quan, "the classics say that the crossbow is fury. It is said that its noise is so powerful that it sounds like fury, and that's why they named it this way," and by using the volley fire method there is no end to the sound and fury, and the enemy is unable to approach. Here he is referring to the word for "crossbow" "nu" which is also a homophone for the word for fury, "nu".