The idea that knots could block evil led to them becoming symbols of protection in Islam, where complex knot designs are sometimes found carved into the walls of palaces. Knotting a beard was thought to frustrate demons. The same symbolism may explain why, in northern Europe, superstitious fishermen used the knots in handkerchiefs – apparently to block bad weather.
In topology, a branch of mathematics, the trefoil knot is the simplest example of a nontrivial knot. The trefoil can be obtained by joining together the two loose ends of a common overhand knot, resulting in a knotted loop. As the simplest knot, the trefoil is fundamental to the study of mathematical knot theory, which has diverse applications in topology, geometry, physics, chemistry and magic. The trefoil knot is named after the three-leaf clover (or trefoil) plant.