Theory of Machine
The two elements of a pair are said to form a higher pair, when they

Are kept in contact by the action of external forces, when in motion
Permit relative motion
Have a surface contact when in motion
Have a line or point contact when in 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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