The first airways to be delineated by radiofrequency were based on the old Low-frequency radio range also called the "Four Course Radio Range" or "A/N" system. The pilot listened for the stronger of the Morse codes transmitters ("· –" for "A" and "– ·" for "N", indicating left or right of the course); the objective was to be centered on course hearing a steady tone (the A and N Morse codes merge to form a steady tone when the receiver is equidistant from both transmitters).