Staedtler Arco Compass Set "reverse navigator" Questions

What is the use of reciprocal scale? The reciprocal scale, also called the reverse scale, is very useful in determining your location on a map.  Reciprocals are easy to get from a baseplate ...

...There are basically two arms to a compass. One arm usually has a point, which you can remove and reverse and or replace with different points for different paper. The other arm has a leg which can ...