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The term "MOS" is used, on a slate, when a scene is filmed without sync sound (or any sound). 

What MOS orginally stood for is still up for discussion.  

The "Mit out Sound" legend 
Some of the early directors in Hollywood, whose first language was German, had trouble with the sound with.  So instead of saying, " we are recording this scene "without sound" the words came out "We are recording this scene mitout sound Because this word corruption, MOS means "without sound"
    MOS may stand for...  
    • Minus optical signal
    • Minus optical sound
    • Minus optical stripe
    • Muted on screen
    • Mute on sound
    • Mic off stage
    • Music on side
    • Motor only shot
    • Motor only sync

Motor Only Sync! Suffice it to say that if someone asserts that it is "Mit out Sound" and yet they have never worked with interlock distributors or multi-duty camera motors. 

Glen Trew: 
The "Mit out Sound" legend is usually attributed to a director, not a sound technician. Now, a director, by his very trade, is not a technologist, and he would have no reason to be aware of the intricacies of how all the technical equipment worked together ... he was just interested in the overall result. 

Hence, that's why I maintain that "Mit out Sound" is descriptive of the *result* (from the point of view of a non-technologist), while "Motor Only Sync" is descriptive of the *process* (from the point of view of a technologist). 

Edited excerpts from discussion thread "MOS original meaning" at  Mars 2001 

Archived discussion threads: 
  • Forum: origin of "mos" (The Cinema Audio Society Discussion Board)

  • mos.txt
The "Mit out Sound" legend hasn't stuck for all these years because it made sense - it stuck because it's funny. 
Today the acronym can stand for  "Mixer Outside Smoking"  :-) 
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 First version of this page 23/3 1997

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