Most inhabitants live in the southernmost part of Hesse between the rivers Main and Rhine. The latter one borders Hesse on the southwest without running through the state. The mountain chain between Main and Rhine is called the Odenwald.
Hesse-Kassel was called the Electorate of Hesse from 1803 on, although since Napoleon, there was nobody to elect anymore. In 1866 it was together with Hesse-Homburg and the duchy of Nassau annexed by Prussia, which established the province of Hesse-Nassau.
Hesse-Darmstadt was called the Grand Duchy of Hesse from 1806 on. It managed to defend its autonomy, because in the War of 1866, it had sided with the winning Prussia and remained one of its allies.
During the Weimar Republic, Hesse-Nassau and Hesse were two administrative regions of Germany. After World War II in 1945, the state of Hesse was established within the United States occupation zone. It by and large combined the former states of Hesse(-Darmstadt) and Hesse-Nassau. Essentially (except for the parts of Hesse on the western banks of the Rhine, which became a part of Rhineland-Palatinate), this was a restoration of the pre-1567 Hesse.