Bennett's condition is a point, not a region

The same four bars and the same four twists throughout; only the length of the two b-bars changes, by up to 6.0 per cent either way. At the condition sin α / a = sin β / b the two legs impose three constraints twice over and the linkage turns; anywhere else one of those coincidences fails, ν drops from 3 to 2, and the mechanism is a structure. The joint graph, the link count and Kutzbach's −2 are identical at every sample on this axis.