A congruence scheme Σ is a
finite sequence of polynomials. A nontrivial equational class K is a representation of
Σ iff the principal congruences in K can be described in a natural fashion
by Σ. In this paper it is shown that a necessary and sufficient condition
for a congruence scheme Σ whose polynomials do not contain constants to
have a representation is that each polynomial in the sequence be at least
binary.