Unfortunately, Pāṇini's rule-based method of linguistic analysis and description has remained relatively unknown to Western linguistics until more recently. Franz Bopp used Pāṇini's work as a linguistic source for his 1807 Sanskrit grammar but disregarded his methodology. Pāṇini's system also differs from modern formal linguistics in that, since Sanskrit is a free word-order language, it did not provide syntactic rules. Formal linguistics, as first proposed by Louis Hjelmslev in 1943, is nonetheless based on the same concept that the expression of meaning is organised on different layers of linguistic form (including phonology and morphology).