This article provides a few interactive animations of the bar linkage, or "leg system", known as the Jansen's linkage, used in Theo Jansen's "Strandbeest" (Dutch for "beach beast"). "Strandbeesten" are large kinetic structures built out of PVC pipes that, propelled by the wind, "walk" over the beach.
If we consider a single "leg system", then each joint in this mechanical linkage has a circular trajectory relative to a joint it links to, because each bar has a constant length. The instantaneous position of a third joint can be calculated as an intersection point of two circles around two connected joints. Considering how the mechanism should function determines which of the two intersection points to choose. Starting with the two joints with known coordinates (the crank and a frame connection) we can determine the third joint's coordinates and this third joint allows a fourth joint to be calculated, then a fifth, etc.
Drag the red dot over the grey circle's circumference to make the linkage move.