Allusions to actual history, geography and current science
In The Hunchback of Notre-Dame, Victor Hugo makes frequent reference to the architecture of the Cathedral of Notre Dame in Paris. He also mentions the invention of the printing press, when the bookmaker near the beginning of the work speaks of "the German pest."
In 2010, British archivist Adrian Glew discovered references to a real-life hunchback who was a foreman of a government sculpting studio in Paris in the 1820s who worked on post-Revolution restorations to the Cathedral.
Allusions in other works
The name Quasimodo has become synonymous with "a courageous heart beneath a grotesque exterior."