Tuojiao Sun (托角榫): How Chinese Corner Spandrels Are Fixed
Tuojiao sun · Corner spandrel joint
Under the corner of many Chinese tables a small carved bracket, a corner spandrel (角牙), fills the angle between leg and apron. It looks decorative, but it also stiffens the corner.
How it is fixed is the tuojiao sun, the corner-bracket joint. It is not one joint but a small group of them.
- Chinese
- 托角榫 · tuōjiǎo sǔn
- Joins
- Corner spandrel to leg and apron (or top)
- Found in
- Tables, stools, chairs, stands
- Signature parts
- Groove in the leg · tongue on the spandrel · loose tenons (桩头)
- Nature
- A group of joints
How it works
Into the leg
A groove is cut in the leg, and a tongue on the spandrel’s upright edge fits into it.
Into the apron
The spandrel’s top edge and the apron (or the top itself) are both mortised, and loose tenons, called 桩头, are planted between them.
A group, not a single joint
That is why sources call it a combination of joints rather than one form. Wang Shixiang drew several ways of fitting spandrels: grooved on both edges, grooved on one and pinned on the other, or tenoned on one and pinned on the other.
How it's made
Step 1Groove the leg
Cut a groove in the leg below the apron.
Step 2Tongue the spandrel
Form a tongue on the spandrel edge that meets the leg.
Step 3Plant the loose tenon
Mortise the spandrel’s top edge and plant a loose tenon in it.
Step 4Mortise the apron
Cut a matching mortise in the underside of the apron.
Step 5Fit the spandrel
Slide the spandrel into the corner.
Step 6Braced
The spandrel stiffens the corner between leg and apron.
Variations
Grooved both sides
The spandrel sits in grooves in both leg and rail.
Groove and loose tenon
One edge in a groove, the other on loose tenons.
Tenon and loose tenon
One edge tenoned, the other on loose tenons.
Can you CNC this?
Possible, with design or setup changes
The bracket’s outline is a 2D profile, and the tongue and the groove in the leg are simple cuts. Any carved relief on the bracket is a separate 3D job and may need a small ball-nose cutter.
General note: a round cutter cannot leave a sharp inside corner, so every recess keeps the tool radius. Round the mating edges or add dogbone reliefs. Sloping faces, undercuts and curves may need special bits, extra setups or more axes.
Free STL downloads (one file per part)
- Leg 腿足 · tuojiao-sun-leg.stl
- Apron 牙条 · tuojiao-sun-apron.stl
- Corner bracket 托角牙子 · tuojiao-sun-bracket.stl
Proportions are illustrative, not measured dimensions. The files are unitless: scale them to suit and check fits and allowances before cutting. Each file is a closed (watertight) solid with its mortises and slots already cut, exported in its assembled position, ready to import into CAM or modelling software. Licence: CC BY 4.0 (credit: Chinese Joinery).
In museum collections
Minneapolis Institute of Art
Related joints
Elongated Bridle Joint
jiātóu sǔnThe leg top is slotted to clamp the apron and its spandrel heads, then tenons into the top.
Mitred-Shoulder Joint
géjiān sǔnWhere a rail meets a post, its outer face is cut to a point that fits a matching notch, so mouldings meet cleanly.
Sliding Locking Key
zǒumǎ xiāoA dovetailed key planted in one part drops into the wide end of a slot and slides to the narrow end to lock.
Questions
What is a tuojiao joint?
The set of joints that fixes a corner spandrel between leg and apron: a tongue into a groove in the leg, and loose tenons into the apron.
Is it decorative or structural?
Both. The spandrel is carved, but it also braces the corner.
Why is it called a group of joints?
Because several separate joints work together to fix one bracket.