There are very few areas of philately that can't be specialized to the max. Take the early postage dues of Romania for instance. Most collectors have just a single example of each of these common stamps but they are known with many perf varieties and especially scarce compound perfs (perforated say 14 at side by 12 at top), some of which are very rare. To specialize in this kind of area one has to know that such varieties in fact exist and then have enough of the material to sift through so as to both find enough material to actually have a collection and also to feel satisfied that you have accomplished something as a collector. This is just an example. Such specialization exists, though, on hundreds of micro areas in our hobby. But for the most part it attracts only a few explorers. In all but a few areas, the vast majority of collectors are content to collect by the catalog and have mostly face different stamps. But there are a few areas of our hobby which collectors have super specialized. Areas such as first issue Chile, Greece Hermes Heads, and US 1c 1851s are specialized way out of proportion to similar issues for the vast majority of philatelic subjects. The reason why it happened this way are a bit obscure and are best understood as caused by the interest of a group of collectors who competed with each other within their subspecialty to take their subject to ever more arcane lengths (this is what is currently happening in the Canal Zone collecting world). But once such super specialization happens it becomes self perpetuating-attracting in turn collectors who want the challenge of such minutia and the gratification of knowing so much about such a small subject.