The great question: Wagner still continue to used natural horn in his orchestral works and operas after Tannhäuser? For example: in his operistic cycle "Der Ring des Nibelungen".
This is a great question! For operas written before Tristan Wagner writes for natural horn and for operas after Tristan he writes for valve horn. Conventions of valve horn writing were not really established so even when Wagner writes for the valve horn, there are still many elements of natural horn performance practices.
