You may see references to "Line A", and occasionally to "Line C" in the FCC regulations and elsewhere. These are imaginary lines running near the Canadian border, used to regulate special conditions that apply in those areas for the purpose of avoiding cross-border interference. There are similar Lines B and D on the Canadian side of the border for the same purpose.
Officially, they are defined in the FCC regulations, 47CFR90.7, Definitions. They are "... taken from Paragraph 2 of Arrangement A contained in the revised Technical Annex to the agreement between the United States and Canada on the 'Coordination and Use of Radio Frequencies Above 30 Megacycles per Second', signed at Ottawa on June 16 and 24, 1965." (https://transition.fcc.gov/oet/info/maps/canline/linea.html)
The definitions are:
Line A. An imaginary line within the U.S., approximately paralleling the U.S.-Canadian border, north of which Commission coordination with the Canadian authorities in the assignment of frequencies is generally required. It begins at Aberdeen, Washington, running by great circle arc to the intersection of 48° N., 120° W., then along parallel 48° N., to the intersection of 95° W., thence by great circle arc through the southernmost point of Duluth, Minnesota, thence by great circle arc to 45° N., 85° W., thence southward along meridian 85° W. to its intersection with parallel 41° N., to its intersection with meridian 82° W., thence by great circle arc through the southernmost point of Bangor, Maine, thence by great circle arc through the southernmost of Searsport, Maine, at which point it terminates.
Line C. An imaginary line in Alaska approximately paralleling the border with Canada, East of which Commission coordination with Canadian authorities in the assignment of frequencies is generally required. It begins at the intersection of 70° N., 144° W., thence by great circle arc to the intersection of 60° N., 143° W., thence by great circle arc so as to include all the Alaskan Panhandle.
See also this interactive map, allowing you to zoom in for greater detail.