Definition

• Contraposition.  The matching opposite polarity; that is, a mirror image. A design with a figure facing right is said to be in contraposition to the same image facing left. One
  of the advantages of the modern pantograph (including the Janvier reducing machine) is its ability to cut a die in contraposition to its model. Of course, all lettering in addition to all detail reverses polarity, and such lettering is termed retrograde lettering. Such unwanted change in polarity would have to be remodeled – a new model would have to be prepared with acceptable lettering.
  How a die is cut in contraposition.  On the die-engraving pantograph the pattern is set to rotate in the normal clockwise manner. The cutting is done in a die block that is set to rotate not in its normal manner, but to rotate in a counterclockwise manner. This will result in the design being engraved in the die block in mirror image.
  In addition to the Pasteur example shown in the adjacent illustrations, another example of contraposition in modern medals is the 1920 Manila Mint Medal whose reverse is in contraposition to the same design on the reverse of the Assay Commission Medal of 1882.
    [sidebar]                                               
                                                            
                   Medals In Contraposition                 
                                                             
       In 1922 the French medallist Georges Henri Prud'homme
   created the portrait plaquette of Louis Pasteur shown on 
   the right above. It was struck by the Paris Mint. In 1958
   the Pasteur Club of Cleveland wanted to honor their idol 
   by issuing a Pasteur Medal for a member recognition       
   program. They wished to utilize Prud'homme's portrait of 
   Pasteur. They asked Medallic Art Company to reproduce    
   the portrait from the 1922 medal and to add new, more    
   appropriate lettering.                                   
                                                            
        The technicians at Medallic Art enlarged the portrait
   from the medal and flipped it, by making Pasteur face the
   opposite direction. This was done on the Janvier pantograph
   which can enlarge as well as reduce – in addition to
   cutting the new design in contraposition to the model!
                                                            
        They cut this enlargement in beeswax, gave the      
   beeswax model to Ramon Gordils, the staff sculptor.   
   He removed the old lettering and modeled the desired
   lettering on the wax model, then made a plaster cast of this.  
   He had to touch up the wax and the plaster (to sharpen   
   the edges of relief), a usual step for any enlarging or  
   casting. A dieshell was made from the plaster, and the   
   dieshell placed on the Janvier pantograph again, this    
   time cutting a new die in normal manner.                 
                                                             
        While this sounds involved, it was done quickly, and
   at less cost than remodeling a new portrait. In addition 
   it saved and reproduced a fine medallic portrait by a     
   famous French artist.                                     

  excerpted with permission from

  An Encyclopedia of Coin and Medal Technology

  For Artists, Makers, Collectors and Curators

  COMPILED AND WRITTEN BY D. WAYNE JOHNSON

  Roger W. Burdette, Editor

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