An analysis is presented which suggests that the ``ankle energy,'' that is, the energy at which the experimental sputtering ratio curve begins to form a low‐energy tail, is the maximum energy which the metallic lattice can propagate in a close‐packed direction. The effect appears to depend only upon the existence of a chaining threshold, and not upon the mathematical model of the sputtering process. The equivalence is demonstrated for the author's statistical model, and for the primary lattice ion model of Kinchin and Pease [G. H. Kinchin and R. S. Pease, Repts. Progr. in Phys.18, 1 (1955)].