Of the twenty-five peacetime army corps of the German Empire, nineteen raised mixed Ersatz brigades in the course of the first few days of mobilization. The logic behind this curious fact can be seen in the following map, which illustrates the policy of exempting army corps on the French frontier, as well as those in East Prussia, from the requirement to raise such formations. (The green flags represent brigades raised by the two army corps of the Kingdom of Saxony. The maroon flag represents the brigade raised by the single army corps of the Kingdom of Württemburg. The dark blue flags of Prussian mixed Ersatz brigades are shaded in ways that distinguish the brigades of one Prussian Ersatz division from those of neighboring divisions.)
While freed from the need to raise mixed Ersatz brigades, frontier army corps mobilized a small number of mobile Ersatz units. The XXI. Army Corps raised a two-battery Ersatz field artillery detachment for the 30th Reserve Division, which served as the mobile reserve for the fortress of Strasburg. The XV. Army Corps raised three such units for that formation. All four of the Ersatz field artillery detachments assigned to the 33rd Reserve Division, the mobile reserve of the fortress of Metz, were formed by the XVI. Army Corps.
As most peacetime army corps mobilized two Ersatz field artillery detachments, it is easy to imagine that the frontier army corps that organized more than two such unit made use of elements provided by other army corps.
For Further Reading:
Some queries/possible minor errors
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1. Regarding the last paragraph, didn't each corps organize 2 Ersatz-Artillery Abteilungs? Going through the orbats at mobilization, each Brigade had 2 Abteilungs, and each came from one of the two divisions in each corps.
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2. Maroon should be Württemberg.
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3. While all Corps on the western frontier did not raise Ersatz brigades, in the east, only the Corps in East Prussia (I. and XX.) were exempt.
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4. Using the flagpole to indicate divisional affiliation is nice.
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5. Was the formation of Ersatz Corps ever considered? They probably had the resources to do it I've been able to find limited documentation of Ersatz-Jäger units and none for Ersatz-Fußartillerie, but as seen with the Reserve Corps, they were not necessary. Since Corps level COs and Staff were a precious resource, and it would be pretty hard to find surplus ones, it would make sense if the idea was either never considered, or considered and abandoned quickly. Also, having a few divisions not beholden to corps provides a level of flexibility for reinforcing and relieving.