Unless specified otherwise, Class E airspace extends from what altitude?

Class E airspace typically extends from 1,200 feet above ground level (AGL) up to, but not including, 18,000 feet mean sea level (MSL) unless otherwise specified. This is crucial for pilots and air traffic controllers because it establishes the vertical limits and helps ensure safe separation from other airspace classifications.

The 1,200 feet AGL starting point allows for transitions of different airspaces, especially as aircraft ascend or descend. The upper limit of 18,000 feet MSL is significant as it marks the beginning of the National Airspace System where Class A airspace starts, which is typically managed under Instrument Flight Rules (IFR).

Understanding these altitudes helps in flight planning and maintaining situational awareness regarding where specific regulations and procedures apply. The other choices provide altitudes that do not align with the standard definition of Class E airspace, which is why they do not represent the correct answer.