Thank you for finally seeing that if spaceship B and the belt were to be co-moving, then it would be very problematic for anyone to claim they experience different gees of acceleration. Yes, it was one of my original premises that spaceship B and the belt would be co-moving. But I started to question that, back when you had me thinking about how frame K would find the belt to be constantly slowing down its horizontal speed, due to increasing effects of time dilation caused by its increasing speed relative to the y axis. Around that time, I asked you if they would remain co-moving or not, and you told that they would. So we both share some of the blame for this confusion. Wait, now you are talking about two spaceships. We are supposed to be talking about spaceship B and the treadmill belt running in the lab. It's probably just a mis-typing, but we really should try to be careful here. We had already agreed on the twin spaceship scenario, so if you are now changing that, I need you to specifically tell me that, please. I had long ago given up on the idea that spaceship A and spaceship B would always remain at the same y coordinate in each of their own frames. I am relying on inertial frame K to set up the scenario where spaceship A and spaceship B always remain at the same y coordinate as each other according to frame K. Surely we can still agree on that, right? If spaceship B and the belt are not co-moving, then I have no problem with them experiencing different gees of acceleration. But I will not be fully happy until I understand how it can be possible that spaceship B and the belt could not be co-moving. It still seems to me that both are equivalent inertial frames, and so it seems they should be co-moving in both the x and y directions of inertial frame K. I suppose that if they are not co-moving in the x direction, then that could lead to them also not being co-moving in the y direction, and that seems to be what you are saying above. But how can it be possible that spaceship B and the belt could not be co-moving in just the x direction of frame K? Let's start with that. So, are you now saying that frame K would find the belt to be constantly slowing down its horizontal speed due to increasing effects of time dilation caused by its increasing speed relative to the y axis, but that would not be the case for spaceship B?? By the way, please do not try to throw a curve ball and say that that spaceship B, being a spaceship, could have rocket thrust in the x direction, whereas the belt, being a treadmill, has no rocket thrust in the x direction. I specifically want all these motions in the x direction of frame K to be inertial, please.