Square joinery teaches you to trust flat reference faces. A chair does not. The legs lean out of the seat, the rails follow the body, and the joints that look wrong on the drawing are the ones that carry the most load. First chairs usually do not fail because of sloppy woodworking; they fail because the joints were cut before the angles were understood, and because the curved parts offer no square face to go back to.
A useful way to build a curved chair with angled mortise and tenon joints is to stage the work around a Z-style frame. In side elevation the frame looks like a Z: rear leg angled back, front leg angled forward, and a rail running between them on the diagonal. Build two of these as side assemblies. Connect them with front and back seat rails. The seat weave is carried by those rails. The Z frame forces you to solve the angled joinery while the stock is still square. Curves come later, and the seat comes last, woven from Danish cord.
Photo by Andrea Bencelova on Unsplash.
Lay Out the Joints While the Stock Is Square
Plane each leg and rail square, and leave it square until the joints fit. Square stock gives you reference faces. Once you cut a curve into a leg, the faces shift, the edges disappear, and layout becomes guesswork.
Curly ash and red oak are both suitable. Ash is tough and finishes with clean shoulders; red oak is more open-grained and a little more forgiving under the rasp.
Mark the joint locations with a knife, not a pencil. On each leg, mark the mortise centerline on the inside face. Use a sliding bevel gauge to establish the side angle. If the rail also angles in plan, mark that angle on the end of the leg. The result is a compound angle. You do not have to cut a compound mortise. A straight mortise is easier to chop accurately, and the tenon can be made to match. The joint is still an angled mortise and tenon joint; the tenon shoulders carry the angle.
Cut the Mortises
Clamp the leg to the bench with the inside face up. Remove most of the waste with a drill, then finish with a sharp chisel. Keep the chisel perpendicular to the reference face and work from both sides so the mortise stays clean. The walls of the mortise do not need to be angled. What matters is that the mortise is straight enough for the tenon to seat without forcing the leg into a twist.
Cut the Tenons to Match
Mark the tenon length from the depth of the mortise and add a little. Saw the cheeks just proud of the line. If the tenon is angled, the cheeks will not be parallel to the rail edges; this is correct. Pare them until the tenon enters the mortise with light pressure.
Now place the rail against the leg and mark the shoulders with a knife directly from the leg. The shoulder line will be angled on the rail. Cut the shoulders carefully, keeping the face clean. A shoulder that rocks over a high spot is the most common source of weak chair joints.
Dry-Fit Before Shaping
Assemble each Z side without glue and set it on a flat bench. The legs and the rail between them should sit without rocking. If the frame twists, correct the shoulders before proceeding. Do not try to pull twist out with clamps. The woven seat can forgive small unevenness, but it will not straighten a twisted frame.
Photo by Rosemary Media on Unsplash.
Shape the Curves
Take the frame apart. Mark the curves on both faces of each leg and rail. Cut close to the line with a bowsaw or a bandsaw. Then work from the outside face inward. Convex curves clean up with a spokeshave; concave curves with a rasp and file. For compound curves, a drill with a sanding spindle is indispensable. It follows the concave shape and blends the transitions where one curve meets another. Keep your strokes short and check the shape with a paper template or by eye from the end. Leave the tenon shoulders alone once they fit.
Glue and Assemble
Before glue-up, dry-fit the entire chair on a flat surface; all four leg bottoms should touch. Number each joint. When the fit is right, apply glue to the tenons and shoulders. Use a caul over any curved part so the clamp pressure spreads. Tighten in stages, checking for twist as you go. Once the glue is set, trim the proud ends and finish-sand the frame.
If the seat rails are to be woven, do not apply a thick film finish over the areas where the cord will cross; Danish cord will wear through it. Finish the legs and the outside of the rails first and weave the seat after.
Weave the Seat Last
Danish cord is a strong twisted paper cord. It is woven while slightly taut, and because it has a little give, it draws the seat rails together and masks tiny irregularities in the frame. The seat is not difficult, but it is exacting.
Start at one front corner and wrap the cord around the seat rail, bringing it across the opening to the opposite rail, around that rail, and back. Continue until you have a complete layer of parallel strands. Then weave the second layer across the first, over and under, to form the Danish pattern. Keep the twist on the outside of the wraps, and keep the strands parallel. Pull firmly, but do not distort the frame. When you finish, coat the cord with shellac or hard wax oil to protect it.
Photo by Decry.Yae on Unsplash.
Useful Mistakes to Avoid
The mistakes that twist a first chair are consistent. Shaping before joinery removes your reference faces. Cutting the tenon shoulders square to the rail when the rail sits at an angle leaves a gap on one side. Clamping a joint that is tight in one spot and loose in another can split the frame. Leave the tenon cheeks a little proud, pare them to fit, and use the seat weave only for minor adjustment.
A chair built in this order, joints first, curves second, and the weave last, will be true even when it is not perfect.