This type of gag is often described in fiction as preventing the subject from speaking, using soft materials like scarves and bandannas. In practice it does not silence the subject very effectively. The long scarf or bandanna is pulled over the subject's mouth and tied at the back of his/her head. Images of OTM gags, usually applied on women, had been prevalent since the film serials of the early 1900s. For this reason, the OTM gag is associated with the typical damsel in distress. It is also sometimes called the "detective gag" because many of its first appearances were in crime serials.
Collection of Over The Mouth Gagged Photoset