We can apply the above recoupling relations to write down the amplitude at each vertex of the decay tree. Connecting the these amplitudes are Lorentz transformations between rest frames: calculating the decay amplitude of a daughter particle requires a rotation into its direction and a boost into its rest frame. The coordinate system is preserved. This procedure results in the decay particle's quantization axis being the same as the momentum direction in the parent's frame, thus forcing the ''m'' quantum number to be equal to its helicity ''λ'' used in the parent's frame.