The four-bar linkage is a common design pattern in mechanical engineering. With 4 rotating bearings and 4 bars, it allows one to constrain motion to a plane. In

The four-bar linkage is a common design pattern in mechanical engineering. With 4 rotating bearings and 4 bars, it allows one to constrain motion to a plane. In particular, along a partucular path both position and angle smoothly change together. Once you start noticing them, you will see them everywhere (bus doors, clamps, etc.). Their available paths are of course limited, but quite varied with interesting tools for design.