Upside-down wedding cakes!!!!!!! Importance and restrictions are alphabetical A(IFR only), B(Controlled w/ radar, but clearance required), C(Radar coverage, just talk to them and have a transponder), D(Just talk to them), E(Technically controlled, still has somewhat restrictive vis & cloud requirements as the above, but it's still a free-for-all), G(No "F" airspace, but G means GO for it!)