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
Have a surface contact when in motion
Have a line or point contact when in motion
Permit relative motion

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Theory of Machine
In elliptical trammels

One pair turning and three pairs sliding
Three pairs turning and one pair sliding
Two pairs turning and two pairs sliding
All four pairs are turning

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