Steven Dutch, Natural and Applied Sciences, University
of Wisconsin  Green Bay);
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For the isometric space groups, we encounter a visualization problem. There is no longer a single principal symmetry direction we can look along. The diagonal 3fold symmetry axes rotate the motif into planes perpendicular to the plane of the diagram so they are seen edgewise. So for isometric space groups, three modes of visualization are employed. First is an oblique drawing of the cubic unit cell with the R motif on a smaller cube. Second is an oblique drawing with stereograms replacing the small cube. The stereograms are drawn in standard crystallographic style without any attempt to represent the projections in perspective. Finally there is a view of the unit cell and stereograms viewed perpendicular to a face. For cases like screw axes where only one or two octants of the cube or stereogram might contain motifs, only those octants are portrayed.
198 P2_{1}3 (+x,+y,+z); (+z,+x,+y); (+y,+z,+x); 

199 I2_{1}3 Origins: (0,0,0;
1/2,1/2,1/2) (+x,+y,+z); (+z,+x,+y; ); (+y,+z,+x); Note that the adjacent pairs of octants in each plane (yellow or green) are not directly related by symmetry. 
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Created 23 July 2001, Last Update 15 December 2011
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