ragged blue line—(U.S.) the Union forces (who wore blue uniforms) in the American Civil war:
- A ragged blue line of four infantry regiments on line and one in reserve moved across the open valley.
thin blue line—(UK) the police; the protective power of the law:
- You’ve heard about the thin blue line that protects property and the owners of property.
See also: all-red line / thin red line.