A gag is usually a device designed to prevent speech, often as a restraint device to stop the subject from calling for help. This is usually done by blocking the mouth, partially or completely, or attempting to prevent the tongue, lips, or jaw from moving in the normal patterns of speech. They are often less effective in reality than in crime fiction. They carry a strong risk of killing the victim by suffocation. The more "effective" a gag appears to be, the more hazardous it is: for example, duct tape is fairly effective but is hazardous if for some reason (e.g., the common cold) the subject cannot breathe freely through the nose.
The use of gags is commonly depicted in crime fiction, particularly in movies, comics, and novels.
Very rarely, courts have been known to gag unruly defendants; Bobby Seale was the most famous case so far. Occasionally a cloth over-the-mouth gag is used not to prevent speech but to keep dust and aerosols out of the lungs.