How best to orient grain for the strongest joints?

[Jay Michaels]

I'm building David Johnson's danish cord bench (featured in Fine Woodworking in 2020). I am building it out of ash. Rather than traditional mortise-and-tenon joinery as David uses, I'm going with the domino.
The stool features one front/back rail, and then a pair of side rails because of how the danish cord is wrapped around them.
My question comes in grain direction for creating the strongest joint / stool. I have two options on how I orient the grain on the top side rails. Should the grain run the same direction as the domino mortise, or should it run perpendicular to the mortise?
I have mocked it up with some pine, but am building the finished project with ash. As you can see from the design, that top side rail will have stress in two directions -- pulled in toward the center of the stool just from the pressure of the cord, and then pushed downward when the stool is being used.
So what makes the strongest stool - grain running parallel to the width of the mortise or perpendicular?

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On that top side rail - should I run the grain parallel to the mortise, like this (OPTION A):
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Or should I run the grain perpendicular to the mortise, like this (OPTION B):
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[Prashun Patel]

It won’t matter, practically.

Orient for aesthetics.

The harder, more critical side is the leg, which, when flared, risks grain runout as yours shows. Again, it’s hard to avoid sometimes and I have made chairs with this condition too - and sometimes there is no issue. But that run out will be a bigger probability of failure than the rail orientation as you’ve indicated.

[Edward Weber]

I agree, orient for aesthetics but I might opt for 2 small tenons rather than one large one. The sidewall on you mortise is quite thin in proportion the the stock.

[Jim Dwight]

I have used my domino successfully to make chairs. But I made all the mortise and tenon joints the same size as the plans called for. That required me to make the loose tenons but I do that anyway. The relatively small size of your stretchers seems like it could be an issue. As you know, the width of the domino mortise increases when the diameter of the bit increases. Using a smaller bit so you stay in the workpiece could be necessary but I would then double them up to get at least the same thickness as the plans call for. If anything, I increase the tenon size, I do not decrease it. If I had to make a stretcher taller to get a sufficient tenon I would do that, even if it affected appearance somewhat. Chairs are under more stress than most things I make and I had one fail (it had traditional mortise and tenons and the failure was not in the joint). So I am more careful now to get them strong enough.

But the orientation question you asked has been answered and I agree with that answer. Regardless you are glueing long grain to long grain, not end grain. But I would arrange it where the tenon grain is aligned with the stretcher grain. In a wide joint it might matter because the wood expands across the grain. If the tenon and stretcher have grain in opposite directions they would try to move against each other. But this is a small piece so the moment will be minimal and I don't think it makes a difference.

[Brian Runau]

I agree, orient for aesthetics but I might opt for 2 small tenons rather than one large one. The sidewall on you mortise is quite thin in proportion the the stock.

Agree, 2 smaller one over the other vs one that is too wide. Brian

[Tom Bender]

Accept the reality that this part is going to flex a little. That flexing will lever the joint open. However if you relieve the inside shoulder it will not have that to pry against. Maybe trim a wedge about 1/32"

[Keegan Shields]

I've not tried this, but you could pin the loose tenons with small dowels to increase the "pull out" strength of your Domino joints. As I understand it, traditional M&T joints have excellent shear strength but relatively poor tensile strength unless they are wedged or pinned.

I guess it depends on how much abuse you think the stool will get.