Phonemes
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.
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.