Earth Prime (Freedom League Designation Earth 1-Prime) is the designation for the universe in which the majority of Freedom City: Play By Post takes place. There are a theoretically infinite number of alternate universes connected to Earth Prime, only a few of which have been discovered by scientists and superheroes from Earth Prime so far. Each newly discovered universe is given a designation by the Freedom League, the organization responsible for interuniversal recordkeeping. Many are also given vernacular names based on their characteristics, such as Earth-Ape for a universe populated by simian versions of Earth Prime residents, or Erde, a universe where the Nazis won World War II and took over much of the world. It should be noted that in each of these universes, as on Earth Prime, these designations are not in regular usage, and the universe is simply referred to as “Earth” or “the universe.” Earth Prime itself is merely an arbitrary designation, created by Dr. Alexander Atom for the relativistic cataloging of universe types.

According to Freedom League research, alternate universes tend to differ from Earth Prime in either one or both of two possible ways: fundamental laws or historical events. Alternate universes also seem to be located “closer to” or “further away from” Earth Prime based on how similar they are in either of these two criteria. As a result, nearby universes with different fundamental laws will often maintain a high level of historical similarity, despite these differences. Similarly, universes with substantially different historical records often tend to hew very closely to the expected physical norms of Earth Prime, including the presence of people who might be expected not to exist given the way history unfolded. Many of these alternate realities or alternate timelines, respectively, have sheaves of alternate universes of their own, differing from each other only in small details.

The “closest” universes to Earth Prime may be so similar as to be nearly indistinguishable except to the veteran dimensional traveler, or may be recent departures from a once-similar universe. Even in worlds with substantially different historical events, the more recent the point of divergence, the nearer that universe appears to remain to Earth Prime. The study of alternate universe travel and theory is still in its infancy, but the Freedom League is confident that as its roster of dimensionally-mobile heroes increases, so will its base of knowledge on the subject.