GAGS & MUZZLES
Strapping a ball into your sub's mouth because their squeals of delight are just a little too loud for the neighbors is certainly one reason to use gags, but far from the only—or even the most common—one. The mouth is a private, sensitive body part, and seizing (or surrendering) control over it is a kind of restraint more intimate even than binding the hands and feet.
Whenever using gags, remember to be very careful about monitoring the wearer's breathing, and always arrange a non-verbal safe signal.