MOS is a standard motion picture jargon abbreviation, used in production reports to indicate an associated film segment has no synchronous audio track. Omitting sound recording from a particular shot can save time and relieve the film crew of certain requirements, such as remaining silent during a take, and thus MOS takes are common on film shoots, particularly when the subjects of the take are not speaking or otherwise generating useful sound.

The abbreviation "MOS" is very peculiar and has no obvious meaning. Two main legends are attributed to the "birth" of the notation:

• MOS may stand for "Minus Optical Stripe," a note from a production sound mixer, notifying recipients that he or she did not expose an optical sound track for a particular scene or take.
• The more popular theory holds that MOS stands for either "Mit Out Sprechen" or "Mit Out Sound," a broken-English phrasing of "Without sound," as a 1920's German-emigré director might have said it. According to this theory, a German director, recently transplanted to Hollywood (probably Ernst Lubitsch, but possibly Fritz Lang or Erich von Stroheim), was asked by a script supervisor how he would like to shoot the next scene of the day. The director responded "Mit Out Sprechen!", and so this was noted as a joke on the production reports and the camera slates for the shot.

References

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Katz, Ephraim. The Film Encyclopedia. 4th Ed. Rev. Kline, Fred and Nolen, Ronald Dean. Collins, 2001.

Film production