Allophony

Two phones are called allophones if they belong to the same phoneme. For instance, in Tok Pisin [t] and [s] are allophones of /t/, and in English [k] and [kʰ] are allophones of /k/.

Allophones are often conditioned by their environment, meaning that one can figure out which allophone is used based on context. For example, the English phoneme /t/ is realized as a tap [ɾ] between vowels in normal speech when not preceding a stressed vowel, for example in the word "butter". In a case like this we can say that the plosive [t] and tap [ɾ] allophones of the phoneme /t/ are in complementary distribution, as every environment selects for either one or the other, and the allophones themselves may be referred to as complementary allophones. Similarly [k] and [kʰ] are in complementary distribution, as [k] mainly occurs in the sequence /sk/, while [kʰ] occurs elsewhere/

By contrast, allophones may sometimes may co-occur in the same environment, in which case they are in free variation. For example, the English word cat's word-final /t/ phoneme may be realized either with an audible release, or as the tongue held in the gesture without being released. These phones, notated as [t] and [t̚] in the IPA, are free variants, as either is allowed to occur in the same position. Similarly [s] and [t] are free variants for some speakers of Tok Pisin.