Liangzhu Duimao (梁柱對卯): Two Beams Lapped Inside a Column and Pinned

The drawing

The third drawing on the same juan 30 plate is labelled 梁柱對卯, 藕批搭掌, 蕭眼穿串: opposed tenons; lapped with a slanted (藕批) cut; pierced holes threaded with a bar. It shows a round column with a square hole, two beam ends facing it, each with a square hole in its end, and a long square bar drawn above them.

How it works

Each beam ends in a tenon that reaches into the column, so the two meet inside it (對卯). There they overlap (搭掌), one over the other. A square bar (串) is then threaded through holes (蕭眼) in the column and both tenons, so neither beam can be pulled out without the other.

Inferred reconstruction (推断复原)

This joint is an inferred reconstruction. It is inferred from: the juan 30 drawing and its label. Inferred, not sourced: how deep the tenons reach, the flat lap (the slanted 藕批 cut is simplified), the direction and size of the bar, and all member sizes.

The drawing

The third drawing on the same juan 30 plate is labelled 梁柱對卯, 藕批搭掌, 蕭眼穿串: opposed tenons; lapped with a slanted (藕批) cut; pierced holes threaded with a bar. It shows a round column with a square hole, two beam ends facing it, each with a square hole in its end, and a long square bar drawn above them. (Yingzao Fashi juan 三十 (plate scan p. 29)).

Each beam ends in a tenon that reaches into the column, so the two meet inside it (對卯). There they overlap (搭掌), one over the other. A square bar (串) is then threaded through holes (蕭眼) in the column and both tenons, so neither beam can be pulled out without the other.

Inferred reconstruction (推断复原). This joint is an inferred reconstruction. It is inferred from: the juan 30 drawing and its label. Inferred, not sourced: how deep the tenons reach, the flat lap (the slanted 藕批 cut is simplified), the direction and size of the bar, and all member sizes.

Inferred reconstruction (推断复原)

This joint is an inferred reconstruction. It is inferred from: the juan 30 drawing and its label. Inferred, not sourced: how deep the tenons reach, the flat lap (the slanted 藕批 cut is simplified), the direction and size of the bar, and all member sizes.

Proportions in the model are illustrative.

Common mistakes

• Driving one beam through as a single through tenon. The plate shows two opposed tenons that overlap inside the column, then a pin through all three.
• Treating lap length, pin size, and the 藕批 slant as book figures. Juan 30 labels 藕批搭掌 and 蕭眼穿串 but gives no measurements; the model uses a flat lap and inferred sizes.
• Leaving out the through-pin. Without the pin the two beams can still pull apart; the drawing places square eyes on the column and both beam ends for the pin.
• Reading it as a single beam-to-column dovetail. One beam dropped into a slot is liangzhu-gumao; this page is two beams meeting through one column.

Related forms

Western and Japanese workshops have relatives that share a geometric idea (scarf, bridle, dovetail, halving, keyed splice) under other names; they are bridges for readers, not claims of descent.

Stay with the Chinese construction on this page before renaming a piece. See also Compare two joints when a look-alike is listed there.