Due to the intermittent motion of movie projectors and movie cameras, the sound cannot be located adjacent to the actual frames it is synced to, but instead must be offset by 21 frames (35mm). Because of this, a married print can not be edited, and thus should only be done when the picture and sound edits have been "locked", or finalized.
Because of the projector and camera's intermittent motion, the audio can't be precisely timed to match the on-screen action. To compensate, the audio is delayed by 21 frames (35mm). This means that a completed print cannot be edited, so it should only be done after both picture and sound edits have been finalized.