Earth's magnetic field  is very similar to the magnetic field of a bar magnetic (left). The bar magnetic has poles at the top and bottom. Iron filings orient themselves parallel to the local orientation of the magnetic field. The field lines are perpendicular to the bar magnetic at the poles.  Similarly, the magnetic field lines for the Earth are perpendicular to the Earth's surface at the Earth's magnetic poles (which are slightly different from the geographic poles) and parallel to the Earth's surface at the magnetic equator. Modified from Press and Sevier (1998).

Magnetic inclination is the angle measured between a horizontal line and the magnetic field line. The inclination is 0° at the magnetic equator and 90° at the magnetic pole. Because of the systematic relationship between magnetic latitude and inclination, we can calculate the magnetic latitude from the magnetic inclinations preserved in fossil magnetism in rocks (paleomagnetism). Modified from Marshak (2001).