Although spanning a relatively short period of time from 1848 until 1868 the political and postal history of the duchies of Schleswig-Holstein is complex. I guess one can say that the duchies were the focal point of many of the dynamics in the German Confederation in the mid 19th century. Not just tensions with Denmark over the supremacy in Schleswig-Holstein but eventually also the struggle for the dominance in the German Confederation between Austria and Prussia.
An added feature in this diagram when compared to other diagrams on StampWorldHistory are the catalog numbers – Michel and Scott – of the stamps used in Schleswig-Holstein in each of the stages in the period from 1848 until 1868. This because the catalogs themselves are not overly helpful when trying to understand the exact postal developments.
A country profile on Schelswig-Holstein elaborating on all the above will be posted in due time. For now for an excellent exposé on the political and postal history of the duchies please refer to the reply by Mike Warre on the German States country diagram, posted June 29, 2015.