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

Permit relative motion
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

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Theory of Machine
Lower pairs are those which have

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

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