His Deutsche Bühnenaussprache is still relevant, though, for practical purposes it has largely been supplanted by other works that employ the IPA (International Phonetic Alphabet), which Siebs' work did not.
Theodor Fontane | Theodor Mommsen | Theodor Herzl | Carl Theodor Dreyer | Thomas Theodor Heine | Theodor Boveri | Theodor Meynert | Theodor Storm | Theodor Loos | Theodor Leschetizky | Theodor Kittelsen | Theodor Eimer | Theodor Schwenk | Theodor Leutwein | Theodor Kirchner | Theodor Heuss | Theodor Estermann | Theodor Busse | Theodor Anton Ippen | Karl-Theodor zu Guttenberg | Karl Theodor Anton Maria von Dalberg | Karl Theodor | Johannes Theodor Reinhardt | Theodor Wertheim | Theodor Uppman | Theodor Reuss | Theodor Reik | Theodor Kullak | Theodor Körner | Theodor-Heuss-Gymnasium Heilbronn |