  When canonical Hamiltonians of local quantum field theories are transformed
using a renormalization group procedure for effective particles, the resulting
interaction terms are non-local. The range of their non-locality depends on the
arbitrary parameter of scale, which characterizes the size of effective
particles in terms of the allowed range of virtual energy changes caused by
interactions. This article describes a generic example of the non-locality that
characterizes light-front interaction Hamiltonian densities of first-order in
an effective coupling constant. The same non-locality is also related to a
relative motion wave function for a bound state of two particles.
