Languages with an extensive morphological structure frequently exhibit well-defined structural layers. The principal division is between derivational and inflectional formations.
- Nida (1949), a pag.98

The derivational formations are usually, but nor always, 'inner layer' formations, and the inflectional formations are usually 'outer-layer' formations.
- Nida (1949), a pag.99

Inflectional formations: 1. Do not belong to substantially the same general external distribution class as the simplest member of the class in question. 2. Tend to be "outer" formations. 3. Tend to be statistically less nomerous. 4. Have inflectional morphemes with more extensive distribution. 5. Exibit no changes in major distribution class membership. 6. May show a grammatical relationship between words.
- Nida (1949), a pag.99