This arrangements of linkages forms the basis of Hart's linkage for a straight line.
Move the points B and C. As you do this notice the lengths FE and FG.
Also notice FE' which is calculated as (AB+BC)/FE. In a sense, this is the "inverse" of FE. Hence, this cell is also called an "inversor".
How does an inversor cell help to generate an exact straight line motion?
Chris Sangwin, 23 December 2006, Created with GeoGebra