Theory of Machine
In a mechanism, the fixed instantaneous centers are those centers which

Moves as the mechanism moves, but joints are of permanent nature
Remain in the same place for all configurations of the mechanism
Vary with the configuration of the mechanism
None of the listed here

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

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