This thread is for Tach, who wants to argue that parallelism is frame invariant: Tach maintains that violating this rule would violate the principle of relativity. I acknowledge that Tach and I have [thread=111112]argued about invariance of zero-angles[/thread] before in a different context (where the angle was between a surface and a velocity). I also acknowledge that Fedris48 [post=3064880]posted a counterproof[/post] to Tach's claim in a concurrent thread which Tach has essentially ignored. In this thread I present a very simple graphical demonstration that parallelism is not frame invariant. Two boxes of the same proper size pass each other. Each box contains a diagonal rod. Here is the rest frame of Box 1: Please Register or Log in to view the hidden image! Here is the rest frame of Box 2: Please Register or Log in to view the hidden image! Note that the each box is length contracted in the other box's rest frame, so the rods are not parallel. Now, here is the frame in which the boxes have equal and opposite velocity: Please Register or Log in to view the hidden image! In this reference frame, The boxes are equally length contracted, and the rods are parallel. Tach, and anyone else interested, does this not adequately demonstrate that parallelism is relative?