Dark Ages

The First Dark Ages separated the Roman Empire from the Middle Ages. We see the beginning of the First Dark Ages in Poul and Karen Anderson's King of Ys Tetralogy.

The Second Dark Ages:

separate the Solar Union from the Stellar Union;
begin several centuries after World War III.

The period from World War III to the beginning of the Second Dark Ages is covered by eleven works from "Marius" to "Brake."

The Third Dark Ages separate the Stellar Union from a period when human beings unaided by machines can cross interstellar spaces by mentally controlling cosmic forces.

The period between the Second and Third Dark Ages is covered by eight works from "Gypsy" to The Peregrine. The period of mentally controlled cosmic forces is covered by one work, "The Chapter Ends," if indeed this work really does belong in the Psychotechnic History. Either way, The Peregrine is the end of an age, like "Brake" before it. According to the Chronology, the Third Dark Ages begin only eighty years later.