Finding a way to synchronize visual and audio tracks was essential to traditional filmmaking because film stock reacts to light, not sound. During a film shoot, the audio track was always recorded by the audio engineer with a separate system on separate media (so-called double-system recording). (For early sound films, playback of the audio track was synchronized during post-production with sound-on-disc techniques, and the audio track was later added directly to the release print with sound-on-film techniques.) Failure to use clapperboards can prevent the film editor from synchronizing the visual images on film footage with the accompanying audio recordings