A gag is usually an item or device designed to prevent speech, often as a restraint device to stop the subject from calling for help and keep its wearer quiet. 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. The more "effective" a gag appears to be, the more hazardous it is: for example, duct tape is fairly effective at keeping one's mouth shut but is hazardous if for some reason (e.g., the common cold) the subject cannot breathe freely through the nose. For this reason, a gagged person should never be left alone.
The use of gags is commonly depicted in soap operas and crime fiction, particularly in comics and novels. It is also often used in movies, such as Raiders of the Lost Ark and its sequel Indiana Jones and the Kingdom of the Crystal Skull.
During the filming of Rope, a camera operator's foot was broken by a heavy camera dolly during an intensive take, and to stop his vocal noises from being recorded on the film he was gagged and hauled out of the studio so that filming could continue without interruption.
One type of gag familiar in fiction, particularly in crime comics and novels, is a suitably sized piece of cloth pulled over the subject's mouth and tied at the back of his/her head. It is sometimes called the "detective gag" because many of its first appearances were in crime serials.
Sometimes a gag is pushed back between the victim's front teeth into the mouth, or with a hard ball in its middle or reinforced by pushing small cloth items into the mouth. This is common in BDSM, but in practice these sorts of gag can usually be got rid of by working the jaws about and/or pushing with the tongue, and they often do not stop the victim from making a loud inarticulate noise to call for help.
Often adhesive tapes are used for improvised gags. A tape gag can cause the skin on the lips to be damaged in its removal or the adhesive can cause chemical burns or reactions.
The word "gag" has come to have various extended meanings, for example: