North-northwest is a bearing of 337.5 degrees, midway between north and northwest. As with the other secondary points, its interest is practical rather than symbolic. It is worth noting that a compass needle does not point to true north but to magnetic north, and the difference between the two, called variation by mariners and declination by surveyors, can be several degrees or more and changes both with location and over time as the Earth's magnetic field drifts; charts are annotated with it, and it must be applied to convert a compass bearing into a true one. Chinese use of the magnetic needle for orientation, initially in geomancy rather than navigation, is attested considerably earlier than in Europe, and the Chinese also recorded the fact that the needle does not point true north, an observation made there before it was made in the west.
North-northwest is one of the thirty-two points of the traditional mariner's compass rose, bisecting north and northwest at 337.5 degrees, part of a subdivision system that grew out of medieval Mediterranean navigation, where portolan charts from at least the thirteenth century onward show compass roses marked with this finer thirty-two-point graduation alongside, and eventually replacing, the coarser eight- or sixteen-point wind roses of classical antiquity. Sailors were traditionally drilled in boxing the compass, reciting all thirty-two points in order from memory, a skill required of apprentice seamen in the age of sail for reading a magnetic compass card quickly and precisely enough to hold a course, a practical navigational discipline rather than a symbolic or cosmological one.