If I wanted to break my button into four, equal rectangular areas then the upper, left area would be defined by 0,0 - 48,13 or 49 pixels across by 14 pixels down. The bottom, left area would be defined by 0,14 - 48,27. That would make the upper, right rectangle be 49,0 - 97,13. Then the last area, the bottom, right, would be 49,14 - 97,27. Good, now let's talk about circular areas.
Circles only require three numbers - the center point and how big the circle is going to be.
Since my picture has even dimensions, I can't specify a true center and have to choose the nearest point. You simply give coordinates for each of the line connecting points and your area is whatever happens to be enclosed within the shape that results. The lines can even cross if you want. You have to define at least three corners or you'll be in big trouble with the map police. Just to be complete - if you're lucky enough to have access to the full power of a web server then you would still do the paper mapping exercise.