Bicycles/Maintenance and Repair/Chains/Removing a Bicycle Chain
The removing or shortening of a bike chain usually requires a chain tool, a tool that can drive a pin out of the chain. Some individuals choose to disassemble and reassemble a derailleur's jockey wheels rather than to cut a chain, for example, when changing a derailleur. Many older bikes, without a derailleur, have a master link that can be opened with a flat-head screwdriver, whereas chains in modern derailleur bicycles more often use a master link like the ones shown in the images below. Sometimes a master link is supplied as a part of a new chain.
With a Master Link 
The SRAM master link and links that resemble it can be opened by pushing the outer plates of the master link together and inward at the same time. This can be done with your hands, or if it's too tight it may require a special tool (see image). Ordinary pliers have been known to work for some, but the process can be painful and uncertain without the proper item. In an emergency, and provided that you do not mind destroying the master link in the process, a chain tool can be used to drive out its pins. These links are compatible with Shimano chains, as long as they are for the same number of gears on the cassette. i.e. 7, 8 or 9 cogs.
Without a Master Link 
If you do not use a master link, it is possible to break a chain with a chain tool (see image). Unfortunately, chain tools are no use at re-inserting straight pins, (unlike Shimano tapered pins), so the important thing here is to avoid removing them all the way; they must stay in the outer plates, even when the rollers are free. Place the chain in the tool and carefully line up the tool's pin with a rivet of the chain; tighten the screw until the pin pushes out far enough so that it still remains generously within the outer plate. If the rivet is pushed all the way out, it is very difficult to reinsert it, and it is likely that a master link will be needed to make the connection. Withdraw the chain tool's screw to break the chain, and if necessary it can be advanced a little more until it can be separated.
Shimano Riveted Chains 
Shimano chains use special tapered rivets that can be guided into the holes in the outer plates. These use a pin driver (chain tool) to both drive out the pins and to push the pins into place. Any pin excess is then snapped off, at a grooved weak point in the pin.
See Also 
- Master Links : The Wikipedia article on master links