VW  CVM 1.1 
The pgg cell.  
Recall that the symmetries for any wallpaper group can all be represented as compositions of a few, selected isometries, called the generators for the group . For example, the group pgg is generated by the halfturn R_{c} and the horizontal glide B shown in the diagram at the right.
Now what happens when we introduce antisymmetries into a wallpaper function based on this group? If some isometries negate the values of our wallpaper function, then others may be forced to do so also.
For example, if we decide to make a function having B as a symmetry
but with R_{c} as an antisymmetry, then
B_{v} would have to be an antisymmetry because
The picture below shows this, with negating symmetries drawn in green. Every positive glide reflection is horizontal, and thus derived from B. All the halfturns negate the pattern, as they are all "cousins" of R_{c}. All the vertical glide reflections are negative.
 
A pgg' wallpaper with its fundamental cell outlined. Negating isometries are in green. 
There are two groups at play here. If you ignore color, you would say that the symmetry group is pgg. If color is taken into account, so that the halfturns no longer count as symmetries, then the group would be pg.
The precise mathematical formulation of this idea involves pairs of groups in this type of relation. The intuitive idea behind the group homomorphisms, central to the formal discussion, is this:
Take a wallpaper group, E, and tag some elements with +1 and the others with 1. This has to be done in a consistent way so that when you compose symmetries, the tag for the composition of the two symmetries is the product of their tags. If you do this, the set tagged by +1 also is a wallpaper group called G. Of course these groups are infinite, but in some sense G has to be exactly half of E.
In the example above, E, the extended symmetry group, is pgg, while G, the actual symmetry group, is pg.
You may wish to skip the details of the proof and go on to the classification result . It lists all the possible E,G pairs and gives a name to the corresponding pattern type. The pattern above is called a pgg'. The name is supposed to make sense because one of the glides in pgg is negating and so gets a prime, while the other is positive.

