A map is a scaled representation of a portion of the earth's surface; and if you can locate yourself on the map, then you can use a compass to determine which direction to travel to arrive at another point on the map. A good topographical map shows three norths: true north -- the direction to the actual northern axis of the earth's rotation; grid north -- the northern orientation of the map's grid lines, sometimes slightly distorted in translating a round earth's features to a flat map's surface; and magnetic north. The difference between grid north and magnetic north is called the declination, and it is different in different places. Before using the compass to navigate, the map reader needs to convert grid directions on the map into compass directions, using something called a declination diagram.
Directions on a compass are represented in two different ways. One way is by quadrant, that is with the compass face divided between north, south, east and west. These quadrant directions can be further subdivided - for example, into northeast or southwest. For greater precision, directions are also subdivided into increment of degrees or mils; and these subdivisions are called azimuths, which are sequenced clockwise on the compass face. There are 360 degrees on a compass, or 6,400 mils.
Well made compasses have a north-seeking arrow on the bar that is well marked for day and visible at night. They also have a glass cover bordered and mounted in a metal ring, called a bezel ring that rotates using the fingers. The bezel ring has a luminous, well marked index point placed on it, so the user can align the index point with the north-seeking arrow point, to aim the compass at a predetermined azimuth. This allows the user to quickly aim the compass, once the correct azimuth for travel has been determined.
Compasses have slits in their covers, usually with a vertical wire strung along the slit, or a notch. These are for aiming. On the opposite side of the compass, there may be a small rear-sighting arm that raises, with a peep sight or slit sight, sometimes augmented with a magnifying glass, through which the user can look, aligning the rear sight with the front sight. In this way, the user can read the azimuth while aiming the compass at a particular terrain feature. For example, if the user sees a prominent mountaintop, she can aim the compass at the mountaintop, using the sights, and read off the compass face what direction the mountain is, relative to her position.