Variation is a characteristic of language: there is more than one way of saying the same thing. Speakers may vary pronunciation (accent), word choice (lexicon), or morphology and syntax (sometimes called "grammar"). But while the diversity of variation is great, there seem to be boundaries on variation – speakers do not generally make drastic alterations in sentence word order or use novel sounds that are completely foreign to the language being spoken. Language variation does not equate with language ungrammaticality, but speakers are still (often unconsciously) sensitive to what is and is not possible in their native tongue. Language variation is a core concept in sociolinguistics. Sociolinguists investigate whether this linguistic variation can be attributed to differences in the social characteristics of the speakers using the language, but also investigate whether elements of the surrounding linguistic context promote or inhibit the usage of certain structures.
Studies of language variation and its correlation with sociological categories, such as William Labov's 1963 paper "The social motivation of a sound change," led to the foundation of sociolinguistics as a subfield of linguistics. Although contemporary sociolinguistics includes other topics, language variation and change remains an important issue at the heart of the field.
Studies in the field of sociolinguistics typically take a sample population and interview them, assessing the realisation of certain sociolinguistic variables. Labov specifies the ideal sociolinguistic variable to
Phonetic variables tend to meet these criteria and are often used, as are morphosyntactic variables, morphophonological variables, and, more rarely, lexical variables. Examples for phonetic variables are: the frequency of the glottal stop, the height or backness of a vowel or the realisation of word-endings. An example of a morphosyntactic variable is the frequency of negative concord (known colloquially as a double negative). Two well-known and frequently studied morphophonological variables are T/D deletion, the optional deletion of the sound /t/ or /d/ at the end of a word, as in "I kep' walking" (Wolfram 1969; Labov et al. 1968); and the ING variable, the optional pronunciation of -ing at the end of a word as -in', as in "I kept walkin'" (e.g. Fisher 1958; Labov 1966/1982; Trudgill 1974).