Theory of Machine
Open pairs are those which have

Surface contact between the two elements when in motion
Point or line 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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Theory of Machine
Oldham's coupling is the

Second inversion of single slider crank chain
Third inversion of double slider crank chain
Second inversion of double slider crank chain
Third inversion of slider crank chain

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