What is complementary distribution in phonology?

• A. A relationship where two sounds co-occur freely in the same environment.
• B. A relationship where two sounds are never found in the same phonetic environment; their allophones occur in mutually exclusive contexts.
• C. A process that inserts vowels to break up consonant clusters.
• D. The rule by which sounds are categorized as vowels.

Answer: (B)

Complementary distribution is when two surface sounds are different realizations of the same underlying sound (an allophone) and they never occur in the same phonetic environment. The choice between them is dictated by context, not by contrast in meaning, so they do not create separate words.

A classic example is the aspirated versus unaspirated voiceless stops in English. At the beginning of a stressed syllable, stops typically come out aspirated (sound with a puff of air). After an initial s in a cluster (as in stopping or “stop”), the same stops appear without that aspiration. The environments in which the aspirated and unaspirated variants occur do not overlap, so these two sounds are predictable variants of one phoneme rather than separate phonemes.

In short, this question’s statement describes that exact situation: two surface sounds that never share an environment because they’re conditioned by context and are allophones of one underlying sound.