Rope may be constructed of any long, stringy, fibrous material, but generally is constructed of certain natural or synthetic fibres. Synthetic fibre ropes are significantly stronger than their natural fibre counterparts, but also possess certain disadvantages, including slipperiness.
Common natural fibres for rope are manila hemp, hemp, linen, cotton, coir, jute, straw, and sisal. Synthetic fibres in use for rope-making include polypropylene, nylon, polyesters (e.g. PET, LCP, Vectran), polyethylene (e.g. Dyneema and Spectra), Aramids (e.g. Twaron, Technora and Kevlar) and acrylics (e.g. Dralon). Some ropes are constructed of mixtures of several fibres or use co-polymer fibres. Wire rope is made of steel or other metal alloys. Ropes have been constructed of other fibrous materials such as silk, wool, and hair, but such ropes are not generally available. Rayon is a regenerated fibre used to make decorative rope.
The twist of the strands in a twisted or braided rope serves not only to keep a rope together, but enables the rope to more evenly distribute tension among the individual strands. Without any twist in the rope, the shortest strand(s) would always be supporting a much higher proportion of the total load.
The long history of rope means that many systems have been used to state the size of a rope. In systems that use the "inch" (British Imperial and United States Customary Measure), large ropes over 1 inch diameter such as are used on ships are measured by their circumference in inches; smaller ropes have a nominal diameter based on the circumference divided by three (rounded-down value for pi). In metric systems of measurement, nominal diameter is given in millimetres. The current preferred international standard for rope sizes is to give the mass per unit length, in kilograms per metre. However, even sources otherwise using metric units may still give a "rope number" for large ropes, which is the circumference in inches.
Rope is of paramount importance in fields as diverse as construction, seafaring, exploration, sports, theatre, and communications, and has been used since prehistoric times. To fasten rope, many types of knots have been invented for countless uses.Pulleys redirect the pulling force to another direction, and can create mechanical advantage so that multiple strands of rope share a load and multiply the force applied to the end. Winches and capstans are machines designed to pull ropes.