Professor Taylor contends that the expression used to describe a course of action so simple as to leave no room for mistakes is plain sailing; that this is nautical in origin in that it derives from a simple or plain system of navigation based upon the use of a simple or plain (manifestly foolproof) chart; that this system of navigation was known originally as plain (simple) sailing—which expression she traces back to Richard Norwood's Doctrine of Plaine and Sphericall Triangles of 1631, and that it was sophisticated into plane sailing in the eighteenth century in the belief—which she holds to be erroneous—that the expression described a form of navigation based upon the use of a plane or flat chart on which the Earth was drawn as if the Earth and oceans lay in one horizontal plane area and not upon the surface of a sphere or, more accurately, ellipsoid; and, finally, that the Admiralty Navigation Manual is in error in teaching mariners that ‘to regard certain small triangles as plane… gives rise to the expression plane sailing, which is popularly referred to as if plane were spelt plain and the sailing free from difficulty’.