Each one of these meaning-distinguishing sounds in a language is described as a phoneme. When we learn to use alphabetic writing, we are actually using the concept of the phoneme as the single stable sound type which is represented by a single written symbol. It is in this sense that the phoneme /t/ is described as a sound type, of which 42 The Study of Language all the different spoken versions of [t] are tokens. Note that slash marks are conventionally used to indicate a phoneme, /t/, an abstract segment, as opposed to the square brackets, as in [t], used for each phonetic or physically produced segment.

An essential property of a phoneme is that it functions contrastively. We know there are two phonemes /f/ and /v/ in English because they are the only basis of the contrast in meaning between the words fat and vat, or fine and vine. This contrastive property is the basic operational test for determining the phonemes that exist in a language. If we substitute one sound for another in a word and there is a change of meaning, then the two sounds represent different phonemes.

The technical terms used in creating those charts can be considered “features” that distinguish each phoneme from the next. If the feature is present, we mark it with a plus sign (+) and if it’s not present, we use a minus sign (−). Thus /p/ can be characterized as [−voice,+bilabial,+stop] and /k/ as [−voice,+velar,+stop). Because these two sounds share some features (i.e. both are voiceless stops), they are sometimes described as members of a natural class of sounds. The prediction would be that sounds which have features in common would behave phonologically in some similar ways. A sound which does not share those features would be expected to behave differently.