Complementary Distribution - in Phonology

In Phonology

Complementary distribution is commonly applied to phonology, where similar phones in complementary distribution are usually allophones of the same phoneme. For instance, in English, and are allophones of the phoneme /p/ because they occur in complementary distribution. always occurs when it is the syllable onset and followed by a stressed vowel (as in the word pin). occurs in all other situations (as in the word spin).

There are cases where elements are in complementary distribution, but are not considered allophones. For example in English and (engma, written with the digraph <-ng> in English) are in complementary distribution, since only occurs at the beginning of a syllable and only at the end. But because they have so little in common in phonetic terms they are still considered separate phonemes.