Refer to figure.
The semi-circular rule is an aviation rule designed to minimize the risk of mid-air collisions, particularly in uncontrolled airspace where pilots may not be under direct air traffic control guidance. The rule provides guidance to pilots on selecting appropriate cruising altitudes based on their aircraft's magnetic heading.

For aircraft flying within these semi-circles, the rule specifies certain altitude ranges to maintain, depending on their magnetic heading:

For VFR:

• 0 to 179 degrees: Fly at odd thousands plus 500 feet (e.g., 3500 feet, 5500 feet, etc.).
• 180 to 359 degrees: Fly at even thousands plus 500 feet (e.g., 4500 feet, 6500 feet, etc.).

For IFR:

• 0 to 179 degrees: Fly at odd thousands of feet (e.g., 3000 feet, 5000 feet, etc.).
• 180 to 359 degrees: Fly at even thousands of feet (e.g., 4000 feet, 6000 feet, etc.).

This rule ensures that aircraft flying on different magnetic headings are at different altitudes, reducing the risk of collision. However, it's important to note that the Quadrantal Rule is just one component of collision avoidance procedures in uncontrolled airspace. Pilots must also use visual and radio communication techniques, maintain situational awareness, and adhere to other regulations and procedures to ensure safe flight operations.

The semi-circular rule is particularly relevant in regions where radar coverage may be limited, such as remote areas or regions with sparse air traffic control infrastructure. By following this rule, pilots can enhance safety and reduce the likelihood of mid-air collisions, contributing to the overall safety of aviation operations.