Two kingdoms made up the core of the Frankish lands, Neustria in the west and Austrasia in the east. They can be very roughly compared to France and Germany, with many caveats. We’ll start with Austrasia, traditionally the larger, stronger of the two, and the homeland of both the Merovingian and Carolingian dynasties.
Austrasia occupied the drainage area of the lower Rhine, including the Main and the Meuse rivers. The Franks did not, however, control the mouths of the Rhine and the Meuse, rather leaving those under control of the Frisians. They did control the coast south of the Meuse, down to approximately the Franco-German border today. When you think of Austrasia, think of Belgium, Luxembourg, the eastward part of the Netherlands, and north-western Germany. The countryside tends to open and flat. The German army took this path in the opening blow of the First World War in their march on Paris (the infamous “right wing”).
While now on Germany’s western border, the city of Aachen (also known as Aix la Chappelle) is near the geographic center of the region. It was the heart of Austrasia, as much as any one place can be called the capital of an area during this time period. Metz was considered the capital prior to the Peppinids.
The Merovingian kings alternately divided their lands between their children, and then reunited them. At times there were two kings, one in Austrasia, and one in Neustria. A civil war that ended in 687 eliminated the last of the Nuestrian kings, and from then on the king ruled from Austrasia. The Austrasian king ruled the united realm, and his court ruled the united lands. It was during this time that a rising family, the Pippinds of Aachen, began to wield more influence. The next century would see Charles Martel, Pepin, and Charlemagne.