Abstract: The size of the eight grades (cai) used for large-scale carpentry (damuzuo) in Song building standards (Yingzao Fashi) is still subject to interpretation. Firstly, the author argues that the grading reflects the commonly used bay widths of the time, because the use of different modules was closely related to the use of differently sized timber in traditional Chinese architecture. And secondly, the author explores a rule for small-scale carpentry that contains text showing up in the wrong order and explains how this mistake may have influenced the formation of module sizes.