Some key adaptations of functional explanation are found in the study of information structure. Based on earlier linguists' work, Prague Circle linguists Vilém Mathesius, Jan Firbas and others elaborated the concept of theme–rheme relations (topic and comment) to study pragmatic concepts such as sentence focus, and givenness of information, to successfully explain word-order variation.[14] The method has been used widely in linguistics to uncover word-order patterns in the languages of the world. Its importance, however, is limited to within-language variation, with no apparent explanation of cross-linguistic word order tendencies.[15]