Previous studies investigating the corner have mostly relied on comparative analysis of Song building standards (Yingzao fashi) with actual buildings, but they have neglected the constructional and mathematical relationships specified in the historical text. By using the rich information on inner-corner component length contained in the Gongxian chapters, this article clarifies the specific meaning of each entry (actual or projection length; side or axial length) and, through re-drawing of the components, demonstrates the inconsistent logic behind the historical data recording. In addition, the article identifies the oblique placement of the large corner beam as Yingzao fashi ’s standard method, because only then was the corner space large enough to accommodate the various corner components. Finally, the article explores the triangular gable space of palatial structures (diange) and less eminent halls (tingtang), as well as their corner beam finishing. All of this is helpful to understand the system of corner rules in official government-sponsored construction of the Northern Song dynasty better.