In physics, all magnets have two poles that are distinguished by the direction of the magnetic flux. In principle these poles could be labelled in any way; for example, as "+" and "−", or "A" and "B". However, based on the early use of magnets in compasses they were named the "north pole" ("N") and the "south pole" ("S"), with the north pole being the pole that pointed north. When it was later understood that opposite poles attract, a terminological dilemma arose: the Earth's North Magnetic Pole and the pole of the magnet that was attracted to it could not have the same polarity. By convention, the "north pole" of a magnet remained defined as the one attracted to the Earth's North Magnetic Pole, and by this definition the Earth's North Magnetic Pole is physically a magnetic south pole. Conversely, the Earth's South Magnetic Pole is physically a magnetic north pole.