protophoneme

Protophoneme is a rarely used term in historical and theoretical linguistics that refers to a hypothetical, pre-differentiated sound category from which later phonemes in a proto-language are reconstructed. It is an abstract unit intended to capture the idea that the earliest stages of a language's sound system may contain simpler or less distinct contrasts, which later evolve into the more complex phoneme inventory observed in daughter languages.

In reconstructive work, protophonemes are invoked to explain how a single early sound category can yield multiple phonemes through processes such as splitting, voicing changes, or changes in phonotactic distribution. A protophoneme might be realized as one sound in one context and as another in a different context, eventually leading to phonemic distinctions like voicing, aspiration, or palatalization across branches. The concept helps model how a language family can display divergent phoneme inventories while sharing a common ancestral stage.

Note that "protophoneme" is not a standard, universally adopted term; many linguists prefer to discuss reconstructions in terms of phonemes at a proto-language level, or simply refer to pre-phonemic stages or phonemic splits without invoking protophonemes. The exact interpretation varies by theoretical framework. The term should be defined clearly in any work to avoid confusion with related concepts such as protophones, which refer to infant vocalizations, or with proto-phoneme reconstructions.