At present, morpheme classes are formed by placing in one class all morphemes which are substitutable for each other in utterances, as 'man' replaces 'child' in 'The child disappeared'.
- Harris (1946), a pag.161

In general, a [morpheme] class may be considered as bound to the level indicated by the number with which it is associated; i.e. it is bound to whatever is substitutable for the symbol-and-number combination that accompanies it in the equations.
- Harris (1946), a pag.179