Where is Western Transdanubia?
I’ve added a line to this map of Hungary. Transdanubia is (approximately) the region to the left, or west of this line. Western Transdanubia is the area shaded in pale blue/lavender.
[T]he western border was always identical with the political boundary of Hungary, therefore it was affected by the territorial changes of the 20th century. Before the Treaty of Trianon in 1920 the present-day regions of Burgenland, Prekmurje and Međimurje were integral parts of Transdanubia. . . . Transdanubia is essentially a Hungarian geographical concept so these areas ceased to be parts of it when they were annexed by neighbouring countries.2
My own research has focused primarily on Vas, Sopron, and Zala counties (the orange, blue, and green areas), and the adjacent region of present-day Burgenland, Austria (all within Western Transdanubia).
Regarding the territorial changes and the Treaty of Trianon, the image below represents the areas that were once a part of Hungary. These lands were lost on June 4th, 1920, at the end of World War I. (The small bit of land marked “4026 km²” is now Burgenland, Austria.)
There are plenty of sites to visit for more information about the history of the Kingdom of Hungary. For “Hungary’s History in a Nutshell,” visit gotohungary.com. For more information on Burgenland, Austria, visit the Burgenland Bunch website.