Foundations describes the ground the world stands on: the planes behind it, the powers in it, the laws of its magic, its peoples as kinds, and the long past that laid it down. None of that changes from one era to the next. What changes is what the peoples make of it, and this book is the first to look at one era of that making: Caeldon in the Medieval Era, around 1,350 CR.

It is the world at a particular moment, a raw age of high fantasy built on old foundations. The rivers of the cradle-web are held by towns that sell safe passage and keep it with armed boats, the holds keep their gates and their seals, the forest edge is still a quarrel fought with axes and arrows, ruin-delvers go down into halls older than any people now living, and beast-hunters, sellswords, and bounty-takers make a living from what waits on the roads; and far beyond the Glassbelt the folds count their stores, the kindreds drive their herds along the scarps, and the harbours sound their channels at slack water. One road crosses between the two halves of the world, and almost no one takes it. This book walks through all of it as a traveller would, and because a traveller needs a guide, it has one: a Human toll-clerk of the Serathic League called Orrin Talloway, whose road-book carries the reader through every region of the atlas and once across the desert and back.

The book does not repeat what Foundations tells. When a chapter here names a god, a school of magic, or a people, it says what that god, school, or people means in this era and points back to Foundations for the rest. The beasts, crops, and stranger beings of the era have their own book, A Bestiary of Caeldon, and the website’s Reference at joetjen.github.io/The-Decachord/ holds every name and date of the setting, entry by entry.

Like the rest of the series, the book invents nothing. Every fact in it has a home in the technical pages of the Decachord; the book gathers them, orders them, and lets the era speak in its own songs, almanacs, and sayings. Where the two disagree, the pages are right.

Later eras will have books of their own, and they will look back on this one as history. Here it is still the present, and its quarrels are still open.