Theory of Machine
Lower pairs are those which have

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

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Theory of Machine
In a mechanism, the fixed instantaneous centers are those centers which

Vary with the configuration of the mechanism
None of the listed here
Remain in the same place for all configurations of the mechanism
Moves as the mechanism moves, but joints are of permanent nature

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