In the 19th century, the term "red light" was associated with the red lanterns carried by railway workers (which the latter left outside brothels when they entered).[dubious – discuss][citation needed]

During World War I, there were many brothels in Belgium and France; authorities marked them with blue lights for brothels for officers, and red lights for other ranks[citation needed].

One of the many terms used for a red-light district in Japanese is akasen (赤線?), literally meaning "red-line." (This has independent origins from the same term in English). Japanese police drew a red line on maps to indicate the boundaries of legal red-light districts. In Japanese, the term aosen (青線?), literally meaning "blue-line," indicates a non-legal district, also exists.

Still others[who?] believe that the term originated from an early 20th century piece of autobiographical writing entitled "Street Walker Unveiled", written by then-reputable New York City high-class prostitute, Nicole Sullivan, who at the time worked for the notorious "mobster" Most Jokes. In the work she mentions how she would "[walk] about the dim streets in the early morning, red light bathed across [her] face."

In the United States during the 19th and early 20th centuries, the term sporting district became popular for legal red-light districts. Municipal governments typically defined such districts explicitly to contain and regulate prostitution.