杜佑
The encyclopedic text known as the "Tongdian" by Du You from 801 CE also provides a description of the volley fire technique: "[Crossbow units] should be divided into teams that can concentrate their arrow shooting.… Those in the center of the formations should load [their 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 [i.e., proceed to the outer ranks] and once they've shot they enter [i.e., go within the formations]. In this way, the sound of the crossbow will not cease and the enemy will not harm us."