 And most importantly, how can you tell which is the moving frame that has no bearing with regard to time because it is assigned and which is the stationary frame for which time has bearing and for which time is not assigned?  The two frames are equivalent. One could choose either one and the properties would be the same. I chose to 'view from the stationary frame' as Einstein named it. The moving frame is the one with velocity v in the X direction of the staationary frame.  The two frames are equivalent. One could choose either one and the properties would be the same. I chose to 'view from the stationary frame' as Einstein named it. The moving frame is the one with velocity v in the X direction of the staationary frame.  