This worksheet shows part of Peaucellier's linkage. This is usually referred to as Peaucellier's Cell.
If you move the point B you will notice that the product of the distances k=OA.OB is constant. Can you prove this is always the case?
Hence, the distance OA=k/OB. As a result, the distance OB is "inverted" and this arrangement is often called an inversor.
Chris Sangwin, 23 December 2006, Created with GeoGebra