The phoneme /t/ has a number of phonetic variants depending on its position in a word. It can be seen that the phonemics symbol/t/ is a cover symbol for a range of different sounds in actual speech. All of the sounds / phones for which /t/ is a cover symbol are referred to as its allophones( sometimes called positional variants, since they occur in specific environments). The positional variants that are transcribed as [t], [t], [t ] are all instances of the same phoneme /t/. it is important to stress that every positional variant is represented by a phone. Every phone is an allophone of some phoneme. Thus, [k] is an allophone of the phoneme /k/ whereas [t] is is the allophone of the phoneme/t/.
The basic idea behind the distinction between phonetic and phonemic transcription systems can be illustrated by considering pairs of words that linguists refer to as minimal pairs: pairs of words that (1) have the same number of phonemes, (2) differ in a single sound in a corresponding position in the two words, and (3) differ in meaning. An example is a pair of words such as fine and vine. They differ in meaning, but phonologically they differ only in the contrast between initial /f/ and initial /v/.
The allophones of a phone can occur in what is called complementary distribution; that is one allophone an occur in a position where the other allophone can never appear. The term complementary distribution is used because the distribution of one allophone is the complement of the distribution of the other. .