From the first time I made these chair joints until now I have made them the same each time in designs that require this type of joint. I have not had one fail yet. Honestly I had wondered for a long time just how the factory made there joinery. Turns out it is the same. I discovered some photos of an antique one that was for sale and the joint had come apart and there was my answer. I don't think it takes a real genius to figure this out but it is nice to have confirmation when you see a factory example of it to confirm you aren't making substandard joinery.
So I will let the pictures speak for themselves especially since I am heading out of town for the weekend and my time is a bit crunched.
By the way I do cut the mortise on the slot mortiser. You could use a router as a substitute but couldn't make any recommendations on how to use one of those things.
Here is an example of an assembled joint from a Møller style bench. You can see the typical thicker portion of the post at the joint height. This helps add more meat to the joint and a shoulder surface for support of racking forces.
Below is how the joint intersects in the middle of the post. The mortises make an X if you viewed a cross section of the post. I place the rail that is shown vertically as the front to back rail and the horizontal as the left to right rail. This way the front and back rails "pin" the side rails in the mortise. I think of it as a sort of back up if the glue joints weaken. There is plenty of glue surface in these joints as well. Surprisingly strong.
Some additional views below.
Above is the joint before shaping. I carve off the corners and then finely rasp the surface to match the shape and thickness of a piece of cord.
I have found no faster way to round down the tenons to match a slotted mortise than a no.6 grain rasp. On that note I prefer round as opposed to square mortises here because I think the joint is less likely to split out when stresses are applied to the joint.
For reference the tenon is 3/8" thick by 1-7/8" high (I think?) and the post are 1-1/2" square at the joint.
Give it a try and let me know what you think.