The Terminal Radar Service Area
Prior to all the airspace you just read about (Class A, B, C, D, E, and G airspace), there were only two types of airpace; controlled and uncontrolled. Controlled airspace was designated as a TRSA (pronounced ter-sa).
These terminal radar service areas have mostly vanished from modern-day sectional charts, but a few still exist (about 5 or 6 of them in the country). They look like the airspace below, with gray lines and a cake design similar to Class B airspace.
These TRSA’s are still useful to us as Private and Instrument Rated Pilots. They act as an approach control service does for Class C and B airports, but are voluntary for pilots to participate (participate means contact the controlling agency). So yes, you can just fly right through this “grey” airspace without talking to anyone, and there are no specific equipment requirements like a radio or transponder to fly through it. To find what frequency you would contact if you wanted to participate in the “radar area” (i.e. you wanted to get VFR flight following), you would look at the side panel of the sectional chart and find the service hours along with the frequencies for that airspace. The side panel looks like the excerpt below: