After you fit the legs on the gothic stool, it's time to move onto the stretcher and tusk tenons.
I left the stretcher longer than necessary and will trim its tenons to length once the stool is completely built.
The shoulders of the tenons on the stretchers will be angled, just like the legs (in this case—7º), so you'll need to keep that in mind as you cut them.
Once the tenons are fitted to the mortises in the legs, you need to cut the mortises for the wedges that will go through the ends of the stretcher and will pin the entire stool together. This creates the tusk tenon joint.
Because the wedge goes through the stretcher sideways, the mortise will be wider on one side of the stretcher than the other.
I used a 5º slope for the mortise—enough of an angle that the wedge won't slide all the way through, but not so much that it wants to pop loose.
Next up: shaping the legs.