The article compares the academic Estonian grammars by Ferdinand Johann Wiedemann, a great figure in the 19th-century Estonian linguistics. More precisely, the article deals with Wiedemann’s grammar of the Võru dialect of South Estonian ”Versuch ueber den Werroehstnischen Dialekt” (1864) and his ”Grammatik der Ehstnischen Sprache” (1875) — a major Estonian grammar focusing on the central dialect of North Estonian but also treating other dialects. First, the article characterizes F. J. Wiedemann’s research methods and starting points. F. J. Wiedemann’s research on Estonian grammar grew out of polemics with the Estonian grammar by Eduard Ahrens, especially its second edition published in 1853. F. J. Wiedemann stressed some features not covered by E. Ahrens’ grammar of the North Estonian coastal dialect, for example, the opposition of three phonological quantities. F. J. Wiedemann applied the model of grammar first used in his grammar of the Võru dialect to his comprehensive grammar of the Estonian language. The grammars by F. J. Wiedemann present many more historical features of the Estonian language than the ones to be found in contemporary Standard Estonian. That is why F. J. Wiedemann’s works continue to be an important source for the study of the historical development of the Finnic languages.