Divergence

The Fallout world is set in a universe that exists on an alternate timeline.

Timeline
The franchise's alternate history stretches from pre-War years, to the "future of the fifties," to the decades following the destruction of the earth by nuclear war. With the goal of exploring ideas revolving around a futuristic, post-nuclear world, the game setting exists in a timeline that diverged after World War II. This fork led to a world where technology progressed quickly while maintaining the cultural norms of the mid-century.

Game development
The environment of the game world after the Fallout timeline diverged provided artists a blank slate to work with in terms of designing their futuristic version of modern day. For the alterative timeline's impact on environmental storytelling, many objects were created to look familiar to what exists in the real world and others are developed as those from the future.

Artists used the divergent timeline as inspiration to maintain a mix of vintage, familiar, modern, and futuristic aspects throughout the game world, and developers were intentional in featuring fictional aspects and real world equivalents appearing both before and after the timeline diverges.

As opposed to purely "fantastical sci-fi designs," developers considered how industrial and product design would have evolved in the alternate universe. Technology developed unevenly in the alternate universe, advanced in some ways and primitive in others, with advancements from real world history having never occurred or if similar, developed much differently. This flexibility allowed artists to depict a world with layers of history and culture on both sides of the timeline's divergence, spanning the generations in both directions from the Great War.