A phoneme is the individual level of sound in speech. Throughout the mid-twentieth century, numerous theories have emerged in the attempt to precisely define the phoneme and provide an accurate model of its use across different languages. Linguistic schools in both the former Soviet Union and the United States developed a standard definition of the term around the 1950s: a phoneme is the smallest phonic (having to do with sound) constituent of a word form, one which cannot be broken down into smaller constituents. A phoneme is thus differentiated from something like a syllable, which can be broken down into constituent phonemes. For example, in the word “cantaloupe”, there are three distinguishable syllables (“can,” “ta,” and “loupe”), because syllables are broken down according to the presence of a vowel. However, each of these can further be broken down into the individuals sounds (phonemes) that make up the syllable (“can” = /c/, /a/, and /n/).
The definition of a phoneme is not the same as that of a letter, however, because there are several phonemes in different languages that are made up of multiple letters acting together. In English, a common, multi-letter phoneme is /th/, such as in the word “this.” The “th” in this word is the first distinguishable sound that the language produces. The theory of phonemes is even more important in a language like Russian, in which individual letters serve the same function as consonant clusters do in English. For example, the Russian letter “ч” (pronounced “ch”) takes the place of a sound that requires two letters in English. Thus, phonemes are distinguished entirely by their contributions to individual sounds in speech, not by how many letters make them up.
An allophone is a various articulation of the same phoneme. In general, each allophone can only occur in speech environments in which none of the other allophones of the same phoneme occur. Allophones are formed through the different enunciative positions the mouth can take. An example of this would be the English words “hot” and “top.” Here, both words consist of the /t/ phoneme. However, the word “top” enunciates the /t/ via complete aspiration of the letter, whereas the word “hot” enunciates the /t/ via a glottal stop (whereby a phoneme is pronounced and heard by the audible release of air from the airstream). Thus, phonemes in any language can have multiple different allophones.