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in common with t. These curves each have a double point at 7 
and therefore intersect at 12 additional points S. Through each of 
these points pass 3 lines ¢’ of the ruled surface (¢’). Hence (¢’)** has 
twelve triple points in the plane t. 
The 20 planes arj and Grim are triple tangent planes for each of 
them contains three rays 4’. 
