Theory of Machine
Rectangular bar in a rectangular hole is the following type of pair

Incompletely constrained motion
Freely constrained motion
Completely constrained motion
Partially constrained motion

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Theory of Machine
In gears, interference takes place when

Pitch of the gears is not same
Gears do not move smoothly in the absence of lubrication
Gear teeth are undercut
The tip of a tooth of a mating gear digs into the portion between base and root circles

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