the study of dialects. Variation most commonly occurs as a result of relative geographic or social isolation and may affect vocabulary or features of grammar or pronunciation or both. Dialectology as a discipline began in the 19th century with the development in western Europe of dictionaries and grammars of regional dialects. Much of the work of dialectology has consisted of gathering information about the types of variation that occur in different dialects and the construction of linguistic atlases showing patterns of distribution for a series of varying features within a language. Such work on the geographic patterns of linguistic variation is also known as linguistic geography.