We used to have a “triangle” route LHR-YVR-SEA-LHR ( before the gripes about codes start that’s London-Vancouver-Seattle-London) on the 747 and 744. As I recall it was pretty much a daily service and went one way round the triangle one day, the other the next etc....the logic was at the time was there wasn’t enough demand to fly a daily terminator to each...once demand picked it each city picked up it’s own dedicated service to/from London.