Theory of Machine
The example of completely constrained motion is a

Motion of a shaft with collars at each end in a circular hole
All of these
Motion of a square bar in a square hole
Motion of a piston in the cylinder of a steam engine

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Theory of Machine
Kinematic pairs are those which have

Point or line contact between the two elements when in motion
Surface contact between the two elements when in motion
Elements of pairs not held together mechanically
Two elements that permit relative motion

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