Application oj the theory on cognate six-bars oj Watt's type makes it possible to link the well-known inver sion mechanisms through mechanical cognation. It is proved that the inversors oj Peaucellier, Hart, and Sylvester are cognates, though they do not aU haî'e the same number oj links and turning-joints. As side-lines, two new inversion linkages and two other straight-line mechanisms are jound. AU oj them are each other' s cognate and produce the same straight line. The presented cognates cover a wide range in sizes.