A simplified procedure to determine whether two sounds represent the same or different phonemes. The cases on the extreme left and the extreme right are those in which the sounds are allophones.

Whenever a user's speech is vocalized for a given phoneme, it is slightly different from other utterances, even for the same speaker. That 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, by being 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 the unconscious freedom to choose the allophone that is used.

There are many allophonic processes in English: lack of plosion, nasal plosion, partial devoicing of sonorants, complete devoicing of sonorants, partial devoicing of obstruents, lengthening and shortening vowels, and retraction.

In addition, the following allophones of /t/ are found in (at least) some dialects of American(ised) English;

However, speakers may become aware of the differences if – for example – they contrast the pronunciations of the following words:

These descriptions are more sequentially broken down in the next section.

However, there may be several such allophones, or the linguist may prefer greater precision than that allows. In such cases, a common convention is to use the "elsewhere condition" to decide the allophone that stands for the phoneme. The "elsewhere" allophone is the one that remains once the conditions for the others are described by phonological rules.

For example, English has both oral and nasal allophones of its vowels. The pattern is that vowels are nasal only before a nasal consonant in the same syllable; elsewhere, they are oral. Therefore, by the "elsewhere" convention, the oral allophones are considered basic, and nasal vowels in English are considered to be allophones of oral phonemes.

In other cases, an allophone may be chosen to represent its phoneme because it is more common in the languages of the world than the other allophones, because it reflects the historical origin of the phoneme, or because it gives a more balanced look to a chart of the phonemic inventory.

An alternative, which is commonly used for archiphonemes, is to use a capital letter, such as /N/ for [m], [n], [ŋ].