The term thin line originated in 1854 when the British Red Coats formed a thin red line during the Crimean War. Their battle formation was the last line of defense similar to how we see police officers as the last line of defense between criminals and law-abiding citizens. The term itself did not gain steam until 1922 when the then NYC commissioner Richard Enright used the phrase during a speech. Since then, it has been popularized by entertainment, the media, and pop culture. It has morphed into several different meanings and other organizations have created their own colors and lines.
There most recognized variations of the thin line flags are:
Thin Blue Line – Police or Law Enforcement
Thin Red Line – Firefighter or Fire Department
Thin Green Line – Armed Forces or Military
Thin Yellow Line – Security and Other Law Enforcement
Thin White Line – E.M.S. or Medical Services
Thin Gold Line – Dispatch
The Thin Line Series at Endgrain Workshop features a Subdued American Flag (black and white flag) with a single thin line under the union of the flag.