Literal red line drawn on a map, document, or surface.
地図に赤線を引いてルートを示した。
I drew a red line on the map to show the route.
Historical abbreviation of 赤線地帯; refers to districts marked by a red line on police maps, associated with prostitution. Now a historical term.
See also: 赤線地帯
A road shown by a red line on cadastral maps; not legally recognized as a public road, and sometimes no longer physically exists. Related to 法定外公共物.
See also: 法定外公共物
この土地の境界には赤線が記されている。
A red line is marked on the boundary of this land.
After the war, areas called red-line districts existed in various places.
Compound of 赤 (red) and 線 (line). The historical red-light district sense comes from the red lines drawn on police maps to designate such areas.