Nice looking table design! So, as Harvey says, it looks like the legs splay in two dimensions. I made a little table like that but with one difference. From your pic, it looks like the apron does NOT splay. The one I made did. I made the apron splay at exactly the same angle as the legs. Anyway, I did a bit of experimenting since I did not know what I was doing. I started with the legs . I did not know how much splay I wanted so I simply drew angled lines (Using my sliding bevel thingy) on the bottom of the legs at, some more or less arbitrary angle, maybe five degrees, and cut that compound angle. Easy. I then set the leg up so see what it looked like. After three or four tries I got a splay I liked, though next time I will probably use less. I cut the top of the leg at exactly the same angles. I then did the same angles on the other thee legs. It is a bit harder to cut the leg at double angle, but I found I could do it with one or two practice tries. Next, since I wanted the apron to splay at the same angle as the legs, I cut perfectly ordinary mortises along the leg and perfectly ordinary tenons on the apron boards to fit those mortises. Surprising straight forward. I then realized that that made the top edge of the apron slanted so I just planed that so it would be horizontal. To that end, I just used the tops of the legs as guide. Now your example does not seem to have the apron splayed. So your mortise will not follow straight along the leg. Rather, to make it vertical you need to make it angled with respect to the leg. I suspect that there is some easy way to figure out exactly how to do that — it is determined by how much splay you have. On the other hand, I learned when working on compound mitered dovetails that the angles are not always what you might think when you are dealing with compound angles. I would just give it a try with cheap wood and simple legs rather than the fancy ones in your drawing. Love to know how it works out.