Figure 2-10c shows a fourbar linkage transformed into a earn-follower linkage by
the application of rule #4. Link 3 has been removed and a half joint substituted for a full
joint between links 2 and 4. This still has one DOF, and the cam-follower is in fact a
fourbar linkage in another disguise, in which the coupler (link 3) has become an effective
link of variable length. We will investigate the fourbar linkage and these variants of
it in greater detail in later chapters.