Clip on dampeners are usually made of steel or plastic and have a soft felt pad which can be easily adjusted to suit the amount of dampening required. They attach in any position around the counter hoop. Clip on dampeners are ideal for the snare drum and the tom toms, but are not usually used for the bass drum.
The further in from the edge that the felt pad is, the more muffled the drum becomes. Most clip on dampeners can also be adjusted to lift up from the drum head for maximum sustain when necessary.