The standard non-trivial mechanism is a four bar linkage. Aptly named, as it is drawn with three bars.
(An imaginary 4th bar is assumed between A and D).
Now if we drag points on the drawing without constraints being set, it certainly does not behave like a mechanism – the “bars” stretch or shrink, which would not be possible in real life, unless they were made of elastic.
In order to have our drawing behave like the mechanism, we need to constrain the lengths of its sides. We also need to constrain the location of points A and D, where the mechanism is attached to a rigid base.
Now if we drag point B, we see the drawing moves in such a way that the lengths |AB|, |BC|, |CD| are conserved, and A and D stay fixed. This is the (interesting) mechanism motion.
Four bar linkages often have an end-effecter rigidly attached to BC, but at some distance from the centerline. We can add this to our model by drawing a triangle, and constraining the lengths of its sides.
