Page "Allophone" Paragraph 5

When a specific allophone ( from a set of allophones that correspond to a phoneme ) must be selected in a given context ( i. e. using a different allophone for a phoneme will cause confusion or make the speaker sound non-native ), the allophones are said to be complementary ( i. e. the allophones complement each other, and one is not used in a situation where the usage of another is standard ).

In the case of complementary allophones, each allophone is used in a specific phonetic context and may be involved in a phonological process.