Red Light District, Amsterdam, Netherlands

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.
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.