Any set of forms which exhibits some phonological similarity may constitute a form class. Such phonological groupings may be based on: 1.Form and number of syllables. [...] 2. Sequence of segmental phonemes. [...] 3. Occurrence of specific segmental phoneme. [...] 4. Occurrence of suprasegmental phonemes. [...] 5. Junctures. [...]
- Nida (1949), Pag. 107

The morphological significance of a form class is frequently manifest by its relevance in the following circumstances: 1. Change of phonemes. [...] 2. Definition of a sequence class by a form class. [...] 3. Definition of a distribution class by a form class. [...] 4. Junctures. [...] 5. Order. [...]
- Nida (1949), Pag. 108