Accuracy is important to making this joint, so before machining your stock, double-check and adjust your tablesaw blade to make it square with the tabletop. Then adjust your jointer's fence dead-on square with its bed.
For these sectional tables, we made all the legs and two sets of rails the same width and thickness. To begin, machine the legs and rail pieces to the same width and thickness before cutting the pieces to length. Save the cut-offs to test your dado-blade height. Cut 45° miters on the ends of each rail without reducing its length.
Next, using a dado stack that's a hair wider than half the width of the project parts, make test cuts in the cut-off scraps until the blade cuts half the part thickness, as shown below. Lock the saw's blade height to prevent slipping.
Now you're ready to begin cutting the joints, starting with the legs.
1. Begin dadoing the legs
Mark the dado start on one leg. Clamp a stopblock to a miter-gauge extension, and make dadoes on two adjacent faces of each leg.
2. Use Rails to mark dadoes
Place one edge of a rail at the edge of a dado on one of the legs. Then use a crafts knife to mark the other edge of the dado.
3. Reset your dado stopblock
Align the score mark with the dado blade, and reposition the stopblock. Rotate the workpiece, dado down, and cut the other half of the dado on the two adjoining faces.
4. Test fit the first full dado
Use cut-off scrap to test the fit before making the remaining cuts on each leg.
5. Rabbet the rail parts
Now, rabbet the mitered ends of the rails. Gradually reposition a stopblock until you rabbet to the edge of the miter. Cut identical rabbets on both ends of each apron part.
6. Check the miter fit
Rails should fit snugly within the dadoes, with the mitered end flush with the corner of the table leg. If they don't fit, you'll need to adjust the joint using a sanding block, as shown in the next step.
7. Sand to fine-tune joints
Test-fit the rail parts in the leg dadoes. If the faces aren't flush, gently sand the rail rabbet with a 100-grit sanding block with abrasive on just one surface.
8. Glue and clamp
Working on a dead-flat surface, apply clamping pressure to where the rails overlap the legs. Check for square at each joint and that all four legs touch the surface. Allow the glue-up to dry for at least four hours.
9. Sand pieces flush
Using a random-orbit sander to avoid cross-grain scratches, sand the rails and legs flush up to 180 grit. Avoid accidentally rounding over the edges and corners. Hand-sand inside corners to 180 grit.