Class G is uncontrolled airspace: airspace that has not been designated Class A, B, C, D, or E (AIM 3-3-1). Operating in Class G does not, by itself, require ATC authorization under §107.41. The Part 107 operating limits still apply in full. Without a nearby structure, the aircraft stays at or below 400 feet AGL. Groundspeed stays at or below 87 knots. Flight visibility, from the control station, stays at least 3 statute miles. The aircraft stays at least 500 feet below the clouds and at least 2,000 feet horizontally from them. A flight at 600 feet AGL over open farmland with no structure is not legal under §107.51.
What Class G is, and what it is not
AIM 3-3-1 describes Class G as the airspace that has not been designated as one of the other classes. It often runs from the surface up to the floor of the overlying Class E, and that floor is commonly 700 or 1,200 feet AGL. Those numbers are Class E floors. They are not a fixed Class G ceiling. Where no other class is designated, the FAA Chart Users' Guide says Class G extends up to 14,500 feet MSL. At and above that altitude the airspace is Class E, with the exceptions the guide states.
Class G can remain under a Class B or Class C shelf where the shelf does not reach the surface. It does not exist inside the lateral and vertical limits of Class B, Class C, Class D, or surface-based Class E. If you are inside those limits, you are not in Class G, even when the aircraft is at 100 feet AGL. The classes that do need prior ATC authorization are summarized on the airspace classes page.
Section 107.41 lists Class B, Class C, Class D, and the surface area of Class E designated for an airport. Class G is absent from that list. "No authorization" is about the airspace. It is not a waiver of the rest of Part 107.
The limits that still apply at 400 feet and below
Section 107.51 is the operating-limitation section, and it does not switch off in Class G.
Altitude, when you are not operating near a structure, is 400 feet above ground level. The keyed wrong answers are 400 feet above the launch point and 400 feet above mean sea level. The datum is the ground below the aircraft. A hill that rises under the flight raises the AGL reference. A number on the altimeter that reads 400 feet MSL can be far above or far below 400 feet AGL depending on the field elevation.
The structure exception is the only height increase inside §107.51(b): within a 400-foot radius of a structure, and no higher than 400 feet above that structure's immediate uppermost limit. Open farmland with no structure does not qualify. The question about 600 feet AGL over that farmland keys a waiver of the §107.51 altitude limit. Sparsely populated ground matters to §107.25, operations from a moving vehicle, and it does not raise the altitude limit. Climbing no faster than 87 knots does not authorize 600 feet either. Eighty-seven knots is the groundspeed limit in §107.51(a), a separate sentence from altitude.
Visibility is 3 statute miles, observed from the control station. Cloud clearance keeps the aircraft at least 500 feet below the clouds and at least 2,000 feet horizontally from them. When several limits apply, the most restrictive one sets the ceiling. A solid deck at 1,200 feet AGL over flat open terrain with no structures shows the comparison. Cloud clearance alone would allow 700 feet AGL, because 1,200 minus 500 is 700. With no structure, the 400-foot AGL ceiling is lower, so 400 feet AGL controls. Climbing to the base, stopping at 1,000 feet, or treating 2,000 feet as a vertical distance misses that comparison. The arithmetic is also on the maximum altitude page.
Section 107.43 still applies next to an airport in Class G. No person may operate a small unmanned aircraft in a manner that interferes with operations and traffic patterns at any airport, heliport, or seaplane base. The absence of an authorization requirement does not make the pattern a drone corridor.
A field under a 700-foot Class E floor
The sectional shows a faded magenta vignette, so Class E begins at 700 feet AGL and the surface underneath is Class G. You are mapping crops. No tower, no silo, no structure within the flight. You plan 300 feet AGL, visibility is well above 3 statute miles from where you stand, and the clouds are high enough that 500 feet below them is still above your altitude. You do not request ATC authorization for the Class G. You also do not climb to 600 feet because the field is empty. Six hundred feet AGL, with no structure, needs a waiver of §107.51. The faded magenta line is not that waiver, and it is not a promise that Class G ends at 700 feet in every part of the country.
If a county airport sits at the edge of the same Class G and its traffic pattern crosses your lines, §107.43 is the rule you brief. You stay out of the pattern and you yield. The airspace class did not create a right to interfere.
The trap
The altitude trap treats 400 feet as a height above the launch point or above sea level. The regulation says above ground level.
The farmland trap treats "open" or "sparsely populated" as permission for 600 feet AGL. The keyed answer is that the flight is prohibited unless you hold a waiver of the §107.51 altitude limit.
The cloud trap uses only the 500-foot number. A deck at 1,200 feet AGL would allow 700 feet if cloud clearance were the only limit. It is not. Over open terrain with no structure, 400 feet AGL is more restrictive, and 400 feet AGL is the highest you may fly.
Class G is also easy to memorize as a band that always stops at 700 or 1,200 feet AGL. Those are common floors of the Class E above you. They are not the definition.
Practice the limit questions on the free practice test. The airspace lessons in the course keep Class G next to the §107.51 numbers so "uncontrolled" does not get misread as "unlimited."