Figure 2-9b shows all the isomers for the simple cases of one DOF with 4 and 6 links.
Note that there is only one isomer for the case of 4 links. An isomer is only unique if the
interconnections between its types of links are different. That is, all binary links are
considered equal, just as all hydrogen atoms are equal in the chemical analog. Link
lengths and shapes do not figure into the Gruebler criterion or the condition of isomerism.
The 6-link case of 4 binaries and 2 ternaries has only two valid isomers. These are
known as the Watt's chain and the Stephenson's chain after their discoverers. Note the
different interconnections of the ternaries to the binaries in these two examples. The
Watt's chain has the two ternaries directly connected, but the Stephenson's chain does
not.