That part of navigation which treats a ship's course as an angle, and the distance, difference of latitude, and easting or westing, as the sides of a right-angled triangle. The easting or westing is called departure. To convert this into difference of longitude, parallel, middle latitude, or Mercator's sailing is needed, depending on circumstances. Plane-sailing is so simple that it is colloquially used to express anything so easy that it is impossible to make a mistake.