The uchrony attempts to describe one period of the past, but by being based on historical postulates completion different from those which we know. They is well there clean works of science fiction, namely to build intrigues being held in a coherent world, resting on postulates.
The Master of the High-Castle, novel of Philip K. Dick, puts in scene a group of characters through the United States of the Sixties, but in a world where the Allies capitulated to the powers of the Axis in 1947: in the USA, while the Germans seized the East coast and the area of the Big lakes, the Japanese manage a zone overflowing largely of California, the remainder of the American territory having a statute of independence.
Pavane, of Keith Roberts, presents a world where invincible Armada Spanish overcame the English ordered by Drake in 1588. The XXème century describes in this novel is well-sure rather strange, in particular as regards technological advancement: the Catholic church, all-powerful, has a temporal power as well as spiritual, but some feudal lords continue secret research in prohibited sciences, chemistry and electricity.