A lattice vibration of a crystal with two atom basis (represented by green and red dots). Pale chains represent the stationary position, while the red and green chain points are deflected: In the $$n$$-th lattice plane, one basis atom with mass $$m_2$$ is deflected by $$u_n$$, while the other basis atom with mass $$m_1$$ is deflected by $$y_n$$. Analogously with the other lattice planes, $$n+1$$, $$n-1$$ and so on.