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
Higher pair
Force-closed pair
Lower pair

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Theory of Machine
The relation l = (2/3).(j + 2) apply only to kinematic chains in which lower pairs are used. This may be used to kinematic chains in which higher pairs are used, but each higher pair may be taken as equivalent to

Two lower pairs and one additional link
One lower pair and two additional links
Any one of these
Two lower pairs and two additional links

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