 A compass needle points to magnetic north, not true north. The difference in these is known as magnetic declination. Therefore, a compass reading must be corrected from magnetic north readings to true north readings. An adjustable declination arrow lets you adjust declination to your current location. There is a screw or other mechanism, usually on the underside of the compass, that allows you to rotate the radiated face of the compass to match the current declination for a particular area. Many compasses will have a lanyard and attached as a small adjustment key used to set the declination. To set the declination, turn the compass over and insert the key into the adjustment screw. Turn the key until the declination indicator is the correct number of degrees east or west of zero degrees. Once the compass has been correctly adjusted for declination, all readings made will be true north readings and need not be adjusted for declination until you move a substantial distance to another location.