Biuniqueness is a requirement of classic structuralist phonemics. It means that a given phone, wherever it occurs, must unambiguously be assigned to one and only one phoneme. In other words, the mapping between phones and phonemes is required to be many-to-one rather than many-to-many. The notion of biuniqueness was controversial among some pre-generative linguists and was prominently challenged by Morris Halle and Noam Chomsky in the late 1950s and early 1960s.
An example of the problems arising from the biuniqueness requirement is provided by the phenomenon of flapping in North American English. This may cause either /t/ or /d/ (in the appropriate environments) to be realized with the phone (an alveolar flap). For example, the same flap sound may be heard in the words hitting and bidding, although it is clearly intended to realize the phoneme /t/ in the first word and /d/ in the second. This appears to contradict biuniqueness.
For further discussion of such cases, see the next section.
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