I am currently taking an Introduction to Crystallography course. After studying and understanding the punctual symmetry groups (2mm, 4/m 2/m 2/m, 32,...), the flat groups and the diverse elements of symmetry that exist: helical axes, sliding planes, symmetry planes, among others.
I have doubts when building the diagram of a certain spatial group. For example, in an exercise, I am asked to construct the diagram of the spatial group Pmab.
Pmab implies the existence of: an ordinary plane of symmetry perpendicular to the crystallographic axis a, a sliding plane of type a perpendicular to the crystallographic axis b, a sliding plane of type b perpendicular to the crystallographic axis c.
Also, Pmab is an abbreviated notation, because, in reality, there are also two helical binary axes parallel to the axes "a" and "b", and a binary axis parallel to the axis "c".
On the other hand, Pmab belongs to the orthorhombic (rhombic) system and derives from the point symmetry group 2/m 2/m 2/m 2/m.
This is all the information I get from the Herman-Maugin notation. If there is something wrong that I have understood, I hope you will tell me.
As far as Wyckoff positions are concerned, I am often asked for them, but I am not quite sure what they are.