Tradition tells us that prayer ropes (also known as "komboskini" in Greek or "chotki" in Russian) were invented when a monk was trying to count his prayers with knots, but the devil kept untying them all. One day, an angel appeared to him and showed him a knot that contains three crosses, and which the devil could not untie.
This is how prayer ropes are made today, using a special, complex form of knot that has the form of the cross in it three times. Each prayer rope takes hours of work. They are indispensible for the saying of the Jesus Prayer, which is at the heart of Orthodox spiritual life.