What measurement system is used for sizing standard AN nuts and fittings?

The correct answer focuses on the fact that standard AN (Army-Navy) nuts and fittings use the imperial measurement system, specifically in inches and often in fractions like sixteenths. This system facilitates consistent sizing for components used in aviation and military applications.

Understanding that AN specifications are based on inch measurements is crucial, especially since these standards have been widely adopted for use in aircraft design and maintenance. The use of inches provides a familiar and standardized means for engineers and technicians to work with these components, enhancing compatibility and safety across different manufacturing processes.

While millimeters are commonly used in many engineering fields, they are not the standard for AN fittings. The options suggesting a combination of measurement systems or millimeters alone do not align with the established standards for AN components, which are rooted in the inch-based system.