The encyclopedic text known as the "Tongdian" by Du You from 801 CE also provides a description of the volley fire technique: "units should be divided into teams that can concentrate their arrow shooting.… Those in the center of the formations should load bows while those on the outside of the formations should shoot. They take turns, revolving and returning, so that once they've loaded they exit proceed to the outer ranks and once they've shot they enter go within the formations. In this way, the sound of the crossbow will not cease and the enemy will not harm us."