The Nine Chapters on the Mathematical Art, Vols. 1-3

Divide by the mu unit of area method of 240 paces to get the number of mu. One hundred mu make one qing a land unit equal to 100 mu.

Li Chunfeng and the others note: This is the beginning of the chapter, so the two methods for qing and mu are specifically raised. The other numbers are not mentioned again, but they can be inferred from this. If a field of 1 mu is 15 paces wide and 16 paces long, and one spreads it out to make 15 rows, then each row is 1 pace wide and 16 paces long. If one cuts it horizontally to make 16 rows, then each row is 1 pace wide and 15 paces long. This means that by spreading or cutting, each part is a square. In total, there are 240 paces. A field of 1 mu has the same number of paces. Judging from this, multiplying width and length gives the accumulated paces, verified by the 240 paces.