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A chain is a series of connected links which are typically made of metal. A chain may consist of two or more links.
Chains are usually made in one of two styles, according to their intended use:
- Those designed for lifting, such as when used with a hoist; for pulling; or for securing, such as with a bicycle lock, have links that are torus shaped, which make the chain flexible in two dimensions (The fixed third dimension being a chain's length.)
- Those designed for transferring power in machines have links designed to mesh with the teeth of the sprockets of the machine, and are flexible in only one dimension. They are known as roller chains, though there are also non-roller chains such as block chain.
Two distinct chains can be connected using a quick link which resembles a carabiner with a screw close rather than a latch.
Uses for chain 
Uses for chain include:
- Bicycle chain, transfers power from the pedals to the drive-wheel of a bicycle, thus propelling it
- Chain drive, the main feature that differentiated the safety bicycle
- Chain gun, type of machine gun that is driven by an external power source, sometimes connected by a chain, to actuate the mechanism rather than using recoil
- Chain pumps, type of water pump where an endless chain has positioned on it circular discs
- Chain-linked Lewis, lifting device made from two curved steel legs
- Chainsaw, portable mechanical, motorized saw using a cutting chain to saw wood.
- Chain steam shipping
- Curb chain, used on curb bits when riding a horse
- Door chain, a type of security chain on a door that makes it possible to open a door from the inside while still making it difficult for someone outside to force their way inside
- Keychain, a small chain that connects a small item to a keyring
- Lead shank (or "Stud chain"), used on horses that are misbehaving
- Lavatory chain, the chain attached to the cistern of an old-fashioned W.C. in which the flushing power is obtained by a gravity feed from above-head height. Although cisterns no longer work like that, the phrase "pull the chain" is still encountered to mean "flush the toilet".
- O-ring chain, a specialized type of roller chain
- Roller chain, the type of chain most commonly used for transmission of mechanical power on bicycles, motorcycles, and in industrial and agricultural machinery
- Snow chains, used to improve traction in snow
- Timing chain, used to transfer rotational position from the crankshaft to the valve and ignition system on an internal combustion engine, typically with a 2:1 speed reduction.
- Ball and chain, phrase that can refer to either the actual restraint device that was used to slow down prisoners, or a derogatory description of a person's significant other
- Bicycle lock (or "Bicycle Chain"), lockable chain
- Security chain, chain with square edges to prevent cutting with bolt-cutters.
- High-tensile chain (or "Transport chain"), chain with a high tensile strength used for towing or securing loads.
- Leg iron chains (Fetters), an alternative to handcuffs
- Chain link fencing, fencing that utilizes vertical wires that are bent in a zig zag fashion and linked to each other
- Chain of office, collar or heavy gold chain worn as insignia of office or a mark of fealty in medieval Europe and the United Kingdom
- Chain weapon, a medieval weapon made of one or more weights attached to a handle with a chain
- Omega chain, a pseudo-chain where the 'links' are mounted on a backing rather than being interlinked
- Pull switch, an electrical switch operated by a chain
- Flat chain, form of chain used chiefly in agricultural machinery
- Supported by log floats, large chains have been used to exclude warships from harbors and rivers e.g., the Hudson River Chain
- Decorating clothing, some people wear wallets with chains connected to their belts, or pants decorated with chains
- Jewelry chain, many necklaces and bracelets are made out of small chains of gold and silver
- Jack chain, a toothed chain used to move logs
- Ladder chain, a light wire chain used with sprockets for low torque power transmission
- Anchor cable, as used by ships and boats, in British nautical usage it is a cable, not a chain
- Chains can also be used as a percussion instrument for special effects, such as in Schönberg's Gurre-Lieder and Janáček's From the House of the Dead
- Chain-shot, a type of ammunition for a cannon, used to inflict damage to the rigging of a sail vessel in naval warfare
- Chain mail, a type of armor consisting of small metal rings linked together in a pattern to form a mesh.
According to the "Complete Guide to Chain", the metal link chain was invented in 225 BC.
See also 
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