《周礼·冬官考工记·筑氏&玉人》
用钲的长度作为两铣之间的距离，再去掉二分就是两鼓之间的距离。
以其钲为之铣间，去二分以为之鼓间。
Take the length of Zheng as the distance between the two mills, and then remove the second part to obtain the distance between the two drums.