A four-bar linkage is a fundamental planar mechanism comprising four rigid links connected by revolute joints in a single closed kinematic chain. Grashof's theorem states that if the sum of the shortest (s) and longest (l) links is less than or equal to the sum of the remaining two links (p + q), at least one link can make a complete 360-degree rotation. In a crank-rocker configuration, the input crank rotates continuously while driving the output rocker in reciprocating oscillation.