The long rails of the table are getting two extra short rails in between for additional support of the table top. These are attached with through mortise and tenon. On assembly time the tenons will be wedged inside the mortices. Because these joints are visible from the outside, care should be taken to cut them properly.
After marking out I first define the outline at the visible face. Cutting though the endgrain fibers with a wide chisel and the chopping out the wood in between. Thus I create a shallow groove.
Repeat at the other side. When both sides of the mortise are defined I drill out most of the waste. After drilling it's a matter of chiseling the rest of the waste. Because everything is angled about 80 degrees, it was somewhat fiddly to get it nice all around. The test result with the tenon inserted doesn't look too bad!