One year after Austria’s defeat in 1866 at Koeniggraetz (Hradec Králóve), the Austrian Empire (Kaiserthum Oesterreich) was divided into two independent entities as far as internal affairs were concerned – Imperial Austria and Royal Hungary (Oesterreich-Ungarn) – by the so called ‚compromise’ [‘Ausgleich`].
The two halves were only linked together by three common institutions: the Ministry of Foreign Affairs, the Ministry of War, and a Ministry of Finance to fund the Dual Monarchy’s diplomatic operations and the army and the navy. The subtle difference between the pre-1867 Imperial-Royal (kaiserlich-koeniglich, abbr. k.k.) and the post-1867 Imperial and Royal (kaiserlich und koeniglich, abbr. k.u.k.) is signified by the word "und" in the German expression.
To complicate matters, institutions that belonged to only one of the empire’s constituent parts maintained their old prefix; thus: "k.k. Post", "k.k. Landwehr", etc.
Because the prefix k.u.k. was only introduced for the army and the navy in 1889, we speak of the k.k. Kriegsmarine before this year and thereafter of the k.u.k. Kriegsmarine.