Stopper knots
Stopper knots do exactly what their name tells. They stop objects from falling of the rope. They are always knots, because they are tied in the rope itself. They form a thickening in the rope to avoid it from slipping through for example a belay device or a Prusik knot.
The most well-known stopper knot is the Figure Eight knot. This one is often used in sports climbing.
