If a ‘sailing’ is defined as a means of finding by calculation, tables or construction, either: (a) the course and distance from one place to another, or (b) the position of arrival after making a given course and distance from a given position, then what is commonly known as ‘plane sailing’ is not in this sense, a sailing at all. ‘Plane’ or ‘Plain’ sailing according to modern as well as early usage involves the arguments, distance, d. lat., departuie and course angle, and in order to solve the problems (a) or (b), d. long., and either meridional d. lat. or ‘middle’ (not ‘mean’) latitude must enter the solution.