Theory of Machine
When the two elements of a pair have a surface contact when relative motion takes place and the surface of one element slides over the surface of the other, the pair formed is known as a

Self-closed pair
Lower pair
Force-closed pair
Higher pair

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Theory of Machine
Open pairs are those which have

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

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