  Let A be an L-structure, X a set of elements of A and &lt; a linear ordering of X (not necessarily definable by a first-order formula). We say that (X,&lt;) is an indiscernible sequence in A if for every natural number n, and all elements a1,…,an,b 1,…,b n of A such that a1 &lt; … &lt; an and b1 &lt; … &lt; bn, the map taking each ai to the corresponding bi is an elementary map. If T is a theory with infinite models, then T has models that are the Skolem hulls (see the entry on  classical logic)  of indiscernible sequences. These models are known as Ehrenfeucht-Mostowski models, after the two Polish model theorists who first carried out this construction in the mid 1950s. These models tend to be the opposite of saturated; we can arrange that very few types over sets of elements are represented among their elements. Some important distinctions between different models of set theory can be expressed in terms of the indiscernible sequences within these models; see the entry on  set theory.