Angles With Parallel Sides
Illustration of angles with parallel sides. "GH is parallel to and lies in the same direction as DE, and HI is parallel to but lies in the opposite direction to EF; hence, angle GHI is the supplement of DEF."
International Correspondence Schools I.C.S. Reference Library Mathematics (Scranton: International Textbook Company, 1905) Geometry & Trigonometry Pg. 6
Courtesy the private collection of Roy Winkelman