Abstract: IT is questionable whether the Lorentz transformations should be applied to any of the experiments under discussion, since accelerations are involved. If they are, S, S′ and S″ in D. W. Posener's letter must be regarded as inertial systems and S′ is then moving with the velocity 2v relative to S″. According to the special theory the transformation can then be applied between any pair of the systems and it is impossible to differentiate between them.