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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.