The prime crew members selected for actual missions are here grouped by their NASA astronaut selection groups, and within each group in the order selected for flight. Two versions of the Apollo Command/Service Module (CSM) spacecraft were developed: Block I intended for preliminary low Earth orbit testing, and Block II which was designed for the lunar landing. The Block I crew position titles were Command Pilot, Senior Pilot (second seat), and Pilot (third seat). The corresponding Block II titles were: Commander, Command Module Pilot (second seat), and Lunar Module Pilot (third seat). The second seat pilot was given secondary responsibility for celestial navigation to keep the CSM’s guidance computer accurately calibrated with the spacecraft’s true position, and the third seat pilot served as a flight engineer, monitoring the health of the spacecraft systems.