The expression 'distributional classes' is used to avoid the possible ambiguity of the term 'form-classes'. The latter is now applied to two fundamentally different sets of forms: not only to distributional classes in the present sense, but also sometimes to classes of forms with particular phonological structure (e.g. CVCV as against CV, or high-low tone as against low-high).
- Nida (1948), a pag.434