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
Two elements that permit relative motion
Point or line contact between the two elements when in motion

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

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