Hi, In the classical theory, there is an absolute rest frame, and every frame moving with constant velocity with respect to it is called inertial frame of reference. The frames that are accelerating with respect to it are called non inertial. To test whether a frame is an inertial, we test Newton's first law. Now in SR, there is no absolute rest frame, so why are there frames that are inertial while others not? In the absence of absolute rest frame, all frames should be completely equivalent. Just to clarify the point. Consider two frames where one of them is inertial, the other frame moves towards the first with an acceleration. Now both frames are completely symmetrical to each other, for each frame, the other is accelerating towards it, so what makes one of them preferable (inertial) over the other?