The molecule below,

crystallizes in the monoclinic space group P21/c, with the cell parameters shown in the upper-left corner of the Java frame.

At this point, the cell contains one molecule; this is the so-called asymmetric unit. The other three molecules in the cell will be related to this one by the space group symmetry. It is helpful to orient the cell so the view is down the b-axis (the b-axis is the cell axis that is perpendicular to the two other cell axes). To rotate the cell move the cursor inside the java frame while holding down the (left) mouse button.

A 2-fold screw axis parallel to b at x = 0, z = ¼ produces a second molecule at -x, ½ + y, ½ - z:

This molecule, however, is in a neighboring cell. To move it into this unit cell, translate it by 1 unit cell length in the a direction:

This molecule is related to the asymmetric unit by a 2-fold axis parallel to b at x = ½ , z = ¼ .

A center-of symmetry at the origin of the cell, produces a third molecule at -x, -y, -z:

This molecule is also in a neighboring unit cell. Translate it by 1 unit cell length in each of the a b and c directions:

This molecule is related to the asymmetric unit by a center-of-symmetry at x = ½, y = ½, z = ½ .

A c-glide perpendicular to b at y = ¼ produces the fourth molecule at x, ½ - y, ½ + z: