Complementary and Free-variant Allophones
Every time a speech sound is produced for a given phoneme, it will be slightly different from other utterances, even for the same speaker. This has led to some debate over how real, and how universal, phonemes really are (see phoneme for details). Only some of the variation is significant (i.e., detectable or perceivable) to speakers. There are two types of allophones, based on whether a phoneme must be pronounced using a specific allophone in a specific situation, or whether the speaker has freedom to (unconsciously) choose which allophone he or she will use.
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.
In other cases, the speaker is able to select freely from free variant allophones, based on personal habit or preference.
Read more about this topic: Allophone