《周礼·冬官考工记·磬氏&车人》
从庇端沿着耒木，而到达首端，以两端之内的直线距离为弦，长六尺六寸的耒木，而弦的长度正好等于一步的长度。
自其庛，缘其外，以至于首，以弦其内六尺有六寸，与步相中也。
From the end of the shelter along the plough wood, and then reaching the head end, the straight line distance between the two ends is the string. A plough wood of six feet and six inches has a string length equal to the length of one step.