Suppose ƒ : X* → X* is a morphism and u,v ∈ X*. For every nonnegative integer n, let zn be the longest common
prefix of ƒn(u) and ƒn(v), and let un,vn ∈ X* be words such
that ƒn(u) = znun and ƒn(v) = znvn. We prove that there is a positive
integer q such that for any positive integer p, the prefixes of un
(resp. vn) of length p form an ultimately periodic sequence having period
q. Further, there is a value of q which works for all words u,v ∈ X*.