There is also a third potential isomer for this case of six links, as shown in Figure
2-9c, but it fails the test of distribution of degree of freedom, which requires that the
overall DOF (here 1) be uniformly distributed throughout the linkage and not concentrated
in a subchain. Note that this arrangement (Figure 2-9c) has a structural subchain of
DOF = 0 in the triangular formation of the two ternaries and the single binary connecting
them. This creates a truss, or delta triplet. The remaining three binaries in series form
a fourbar chain (DOF = 1) with the structural subchain of the two ternaries and the single
binary effectively reduced to a structure which acts like a single link. Thus this arrangement
has been reduced to the simpler case of the fourbar linkage despite its six bars. This
is an invalid isomer and is rejected. It is left as an exercise for the reader to find the 16
valid isomers of the eight bar, one-DOF cases.