Object A goes horizontal line and object B diagonal line in 45 degrees angle (Fig 1). A and B have the same velocity ##v## in x-direction. In y-direction, B has velocity ##v##, A has none. The magnitude of ##v## is not very important, but the total speed of B must be below ##c##. In their own rest frames, A and B are square-shaped and of same size. In A's rest frame (Fig 2), horizontal line is moving through A from right to left. In B's rest frame (Fig 3), diagonal line is moving through B from top-right corner to bottom-left corner. The problem (Fig 4). We are in A's rest frame. I think the following three conditions must be met: - B is length contracted in y-direction, but has its proper length in x-direction, because B is moving up with some velocity and there is no velocity in x-direction - Diagonal line's angle is more than 45 degrees, due to length contraction, because the line itself is moving to left. - Diagonal line must go through B so that it goes from top-right corner to bottom-left corner, as it does in B's rest frame, because it's absolute which parts of B meet with diagonal line and which not. Putting these three together, I don't see how they all could be true. B is too shallow to give the diagonal line the room it needs. There is no math yet, but before going into that, I'd like some comments if anyone finds this interesting and worth further effort. Of course if you see this is based on a simple mistake, please point that out, so I don't have to struggle with my misunderstandings anymore. Thanks.