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.
One familiar type of gag in fiction, particularly in crime comics and novels, is a suitably sized piece of cloth pulled over the subject's mouth (and sometimes also the nose) 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. It is sometimes called an "over the mouth" (OTM) gag.
Sometimes a gag is shown pushed back between the victim's front teeth into the mouth ('cleave gag'), or with a hard ball in its middle ('ball gag') or reinforced by pushing small cloth items or even underwear into the mouth ('stuff gag'). 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.
Another most common type of gag in working practice is an over the mouth (OTM) gag of duct tape. A tape gag is, as the name suggests, a type of gag that involves the use of sticky tape. The most commonly used types are duct tape, gaffer tape and PVC tape from two to three inches wide. Tape gags are the simplest gags to apply to someone. If the captor doesn't want the gag to fall off, he/she must wrap a long strip of tape around the lower part of the victim's head, covering the victim's mouth while ensuring that the gag will not slip off. A strip from ear to ear under the jaw helps to restrict jaw movement, making the gag more effective. On some occasions, a captor may add a comical touch to the gag by applying two strips in the form of an "X".
Note that a tape gag can cause the skin on the lips to be ripped off. It can also irritate the lips and cause fever blisters in those who have dormant fever blisters or cold sores. Tape gags can also rip hair off when wrapped around the head. The longer the tape is left on, the harder it will be to remove it from the skin.
For the use of gags in a BDSM context, see gag (BDSM).
The word "gag" has come to have various extended meanings, for example: