A prayer rope (Greek: κομποσκονι, Russian: , Romanian: mtnii, Serbian: , Bulgarian: ) is a loop of knots, usually made of wool but sometimes of wood, that is used during praying to keep track of the number of prayers which have been said.
The rope is usually used with the Jesus Prayer: "Lord Jesus Christ, Son of God, have mercy on me, a sinner." Historically it typically had 100 knots, although prayer ropes with 300, 50, or 33 knots or, less commonly, 250 or 12 can also be found in use today. There is typically a knotted cross at one end, and a few beads at certain intervals between the knots. "The purpose is to help us concentrate, not necessarily to count." 
Its invention is attributed to St. Pachomius in the fourth century as an aid for illiterate monks to accomplish a consistent number of prayers and prostrations. Monks were often expected to carry a prayer rope with them, to remind them to pray constantly in accordance with St. Paul's injunction in I Thessalonians 5:17, "Pray without ceasing."
In some Russian Orthodox service books, certain services can be replaced at need by praying the Jesus Prayer a specified number of times, anywhere from 300 to 1,500 times depending on the service being replaced. In this way prayers can still be said even if the service books are unavailable for some reason. The use of a prayer rope is a very practical tool in such cases, simply for keeping count of the prayers said.
Another form of prayer rope was formerly in use in Russia, and is still preserved among the Old Believers. It is called lestovka ("ladder"), and is arranged asymmetrically. Whereas the more common 100-knot prayer rope is divided into four sets of 25 knots each, separated by larger knots or beads (dividers), the lestovka consists of counters consisting of loops of cloth or leather often containing short lengths of small-diameter dowel, arranged in groups as follows: 12 (for the number of the Apostles); 39 (for the weeks of the pregnancy of the Theotokos); 33 (for the years of Christ's life on earth), and 17 (for the number of prophets). These sections are separated by dividers larger than the counters, and there are three further divider-sized counters at each end, for a total of nine such large counters (for the nine ranks of angels); thus there are a total of 101 counters plus nine large ones. Where the ends join, they are sewn to four triangular leaves (for the four Gospels) sewn together two and two, the upper pair overlapping the lower. The lestovka is used with the Jesus prayer, but also for counting litany responses, which will often total 12 or 33; for this purpose it is better suited than the more familiar variety of prayer rope.