The rectangular shape provides straight edges and square angles for plotting and triangulating on the map. In general, bezels should be large to allow use while wearing gloves - the larger size also improves accuracy. Black bezels with sloping sides are preferred. Marks on the bottom of the interior of the circular compass housing, marked with parallel 'meridian lines', which rotate with the bezel when it is turned. The meridian lines run parallel to the north-south axis of the bezel, however turned.
Meridian lines are necessary for plotting and triangulating on the map.
A 3 to 4-inch base plate. A longer straight edge makes map work easier. In some cases, steadying the compass needle can be difficult. Recommended compasses Compasses must have the first 6 features listed, including a declination adjustment mechanism. Don't buy a new compass without this declination adjustment feature.