Prequels

A future history series, designed to be read in chronological order of fictitious events, is not necessarily written in that order. Some sections can be written backwards. Thus, in Poul Anderson's Technic History:

the only pre-FTL story, "The Saturn Game," was written as a prequel to everything else;

"How To Be Ethnic In One Easy Lesson," featuring Adzel as a student on Earth, was written as a prequel to the trader team series, featuring Adzel as a member of the team;

"Day of Burning," about the trader team on Merseia, was written as a prequel to the Dominic Flandry series, featuring the Merseians;

within the Dominic Flandry series, the Young Flandry Trilogy (see image) was written as an extended prequel to the Captain Flandry series;

The Day Of Their Return was written not only as a sequel to the third Young Flandry novel but also, because it introduces Aycharaych, as a prequel to the second Captain Flandy story, "Honorable Enemies," which had originally introduced Aycharaych;

The Day Of Their Return also introduces Chunderban Desai who, appropriately, instructs Flandry in historical theory in the first of the later Flandry novels, A Knight Of Ghosts And Shadows.

Thus, a fictional history is constructed by writing both forwards and backwards.