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Parallelogram
A parallelogram is a four-sided geometric form, in which opposite sides are of equal length. A rectangle is a parallelogram in which the angles are 90 degrees.
A parallelogram linkage with pivots at the four corners allows the angles to change as it flexes, but the facing sides always remain parallel to one another. This is a very useful mechanism for derailers, because it allows the derailer cage to move sideways while remaining parallel to the chain, and without the friction that would result from a sliding motion, as used on older plunger-type**** derailers.
This type of linkage is also used in some V brakes to provide a more nearly horizontal motion to the brake shoe, and to keep the shoe angle constant.
