A ball gag is a round gag that fits in the mouth. It is held in place with a strap that passes through the center of the gag. It's usually made of plastic or rubber with a strap made of leather or vinyl.
More About Ball Gag
A ball gag fits behind the teeth, and the straps go around the head to hold it in place. It renders the wearer's speech unintelligible, but it is often used more for humiliation than silence because it causes drooling and strange facial expressions.
Some ball gags, called wiffle gags, have holes in them to improve airflow and therefore safety. Using a gag always poses some risk of asphyxiation or choking for the wearer, however, so it's important for couples to come up with a safe signal before play. A person wearing any sort of gag should never be left alone.
