The inventory or the set of elements at a given stratum is discrete from the set of entities found at any other stratum. For example, the set of phonemes is disjunct from the set of entities which one finds at the morphemic stratum. Just as we don't find any feature which is in some context a semantic feature and in another a phonological, so don't we find that kind of thing between any two strata in a stratificational system. With such a structure the use of mutation rules is incompatible. Instead we have realization, whereby elements of one stratum are realized by elements of a lower stratum.
- Lamb, Sydney M. 2004. Language and Reality: Selected Writings of Sydney Lamb. London: Continuum.