These linkages are four-bar Grashof crank-rockers where

s + l = p + q

where

s = length of the shortest link

l = length of longest link

p and q are the lengths of the two remaining links.

Since the sum of the shortest and longest links are equal to the sum of the other two links, a change point condition is reached when all links become collinear. In this case, the output link may change directions. Three different crank-rockers exist as singular linkages.