I’m reading a charming, well-loved 1965 Book Club edition of Dune from the local library. We take Space Opera for granted today, but observe how it needed to be described back then. Inside flap copy, after some standard stuff about Duke Leto and the great rival house of Harkonnen: (italics are from original source are shown in bold here)

A page of medieval history? Not quite. Duke Leo Atreides is moving from a planet, which he owns, to another planet, which has been given in exchange. The Emperor, Shaddam IV, is Emperor of the known Universe, not a country.

I’m guessing that printings from later decades changed this up somewhat. Dune went on to win the inaugural Nebula Award for Best Novel, as well as a Hugo, both in 1966.