The degrees of freedom of a planar mechanism from the Grübler-Kutzbach count, and where the count fails (redundant constraints, the parallelogram). The four-bar linkage: ground, crank, coupler and rocker, the Grashof condition and the four inversions (crank-rocker, double crank, double rocker, change point). The transmission angle as the measure of how well a linkage transmits force, and its limit, the toggle position, which Over-centre mechanisms puts to use. Coupler curves and the idea of kinematic synthesis, finding the link lengths for a required motion, with a three-position synthesis by hand and with pylinkage; the Klann walking linkage as a six-bar example. Example: the bonnet hinge of a car.