The Nelson-Oppen combination method can be used to combine decision procedures for the validity of quantifier-free formulae in first-order theories with disjoint signatures, provided that the theories to be combined are stably infinite. We show that, even though equational theories need not satisfy this property, Nelson and Oppen's method can
be applied, after some minor modifications, to combine decision procedures for the validity of quantifier-free formulae in equational theories. Unfortunately, and contrary to a common belief, the method cannot be used to combine decision procedures for the word problem. We present a method that solves this kind of combination problem. Our
method is based on transformation rules and also applies to equational theories that share a finite number of constant symbols
