There were two Hesses in 1866, Hesse-Cassel and Hesse-Darmstadt, the Grand Duchy of Hesse.

Hesse-Cassel was annexed by the Kingdom of Prussia in 1866, along with the Kingdom of Hanover, to have a corridor between two groups of Prussian possession. The parts of Hesse-Darmstadt north of the Main were kept by the Grand Duke in 1866 and became part of the North German Confederation led by Prussia. The Grand Duke also kept the parts of Hesse-Darmstadt south of the Main, which remained outside of the North German Confederation.

there were several precedents for this in the history of the German Confederation and of the Holy Roman Empire.

See this question:

https://history.stackexchange.com/questions/35773/how-did-member-states-of-the-holy-roman-empire-justify-only-including-part-of-th[1]

And my answer to this question:

https://history.stackexchange.com/questions/34309/which-european-nation-had-the-most-kings-in-the-18th-century/34372#34372[2]

In 1871 all of Hesse-Darmstadt, still ruled by the Grand Duke, became a state in the German Reich.

https://en.wikipedia.org/wiki/Grand_Duchy_of_Hesse[3]

The answer of Pieter Geerkens explains Bismark's main diplomatic goals and the various agreements he made to achieve those goals in the peace treaty, agreements which resulted in part of Hesse-Darmstadt being in the North German Confederation and part of it remaining outside for over four years.

There were two Hesses in 1866, Hesse-Cassel and Hesse-Darmstadt, the Grand Duchy of Hesse.

Hesse-Cassel was annexed by the Kingdom of Prussia in 1866, along with the Kingdom of Hanover, to have a corridor between two groups of Prussian possession. The parts of Hesse-Darmstadt north of the Main were kept by the Grand Duke in 1866 and became part of the North German Confederation led by Prussia. The Grand Duke also kept the parts of Hesse-Darmstadt south of the Main, which remained outside of the North German Confederation.

There were several precedents for this in the history of the German Confederation and of the Holy Roman Empire.

See this question:

https://history.stackexchange.com/questions/35773/how-did-member-states-of-the-holy-roman-empire-justify-only-including-part-of-th[1]

And my answer to this question:

https://history.stackexchange.com/questions/34309/which-european-nation-had-the-most-kings-in-the-18th-century/34372#34372[2]

In 1871 all of Hesse-Darmstadt, still ruled by the Grand Duke, became a state in the German Reich.

https://en.wikipedia.org/wiki/Grand_Duchy_of_Hesse[3]

The answer of Pieter Geerkens explains Bismarck's main diplomatic goals and the various agreements he made to achieve those goals in the peace treaty, agreements which resulted in part of Hesse-Darmstadt being in the North German Confederation and part of it remaining outside for over four years.

There were two Hesses in 1866, Hesse-Cassel and Hesse-Darmstadt, the Grand Duchy of Hesse.

Hesse-Cassel was annexed by the Kingdom of Prussia in 1866, along with the Kingdom of Hanover, to have a corridor between two groups of Prussian possession. The parts of Hesse-Darmstadt north of the Main were kept by the Grand Duke in 1866 and became part ofthe North German Confederation led by Prussia. The Grnad Duke alos kep the parts of Hesse-Darmstadt south of the Main, which remained outside of the North German Confederation.

there were several precedents for this in the history of the German Confederation and of the Holy Roman Empire.

In 1871 all of Hesse-Darmstadt, still ruled by the Grand Duke, became a state in the German Reich.

https://en.wikipedia.org/wiki/Grand_Duchy_of_Hesse[1]

The answer of Pieter Geerkens explains Bismark's main diplomatic goals and the various agreements he made to achieve those goals in the peace treaty, agreements which resulted in part of Hesse-Darmstadt being in the North German Confederation and part of it remaining outside for over four years.

There were two Hesses in 1866, Hesse-Cassel and Hesse-Darmstadt, the Grand Duchy of Hesse.

Hesse-Cassel was annexed by the Kingdom of Prussia in 1866, along with the Kingdom of Hanover, to have a corridor between two groups of Prussian possession. The parts of Hesse-Darmstadt north of the Main were kept by the Grand Duke in 1866 and became part of the North German Confederation led by Prussia. The Grand Duke also kept the parts of Hesse-Darmstadt south of the Main, which remained outside of the North German Confederation.

there were several precedents for this in the history of the German Confederation and of the Holy Roman Empire.

See this question:

https://history.stackexchange.com/questions/35773/how-did-member-states-of-the-holy-roman-empire-justify-only-including-part-of-th[1]

And my answer to this question:

https://history.stackexchange.com/questions/34309/which-european-nation-had-the-most-kings-in-the-18th-century/34372#34372[2]

In 1871 all of Hesse-Darmstadt, still ruled by the Grand Duke, became a state in the German Reich.

https://en.wikipedia.org/wiki/Grand_Duchy_of_Hesse[3]

The answer of Pieter Geerkens explains Bismark's main diplomatic goals and the various agreements he made to achieve those goals in the peace treaty, agreements which resulted in part of Hesse-Darmstadt being in the North German Confederation and part of it remaining outside for over four years.