Cartoon of the formation of CTRs by increasing the out-of-plane lattice constant to infinity 

This animation takes a 2D crystal (left) and its diffraction pattern (right) and extends the vertical (out-of-plane) lattice constant c, causing the diffraction pattern to be compressed in the corresponding direction c*, until, for c = infinity (i.e., an isolated 1D plane of atoms), the diffraction peaks merge into a set of continuous rods. This line of argument is often used in explaining CTRs in low-energy electron diffraction (LEED) and reflection high-energy electron diffraction (RHEED), for which the electrons only interact significantly with the topmost atomic layer. Although this argument is easy to follow, it does not accurately represent CTRs as observed in surface x-ray diffraction, as the penetration depth is always at least a few monolayers, and often many hundreds.