Hermann-Mauguin symbols

The Hermann-Mauguin (or international) notation is the one most commonly used in crystallography, and consists of a set of up to four symbols.

The first one describes the centering of the Bravais lattice: P (Primitive) or C (Centered on a single face) lattices are possible in 2D. The next three describes the rotation axis normal to the plane (1,2,3,4 or 6) and additional mirror or glide lines. For example p3m1 indicates that the unit cell is the smallest possible (Primitive), m indicates a mirror line parallel to the long diagonal of the unit cell whereas p31m indicates that the mirror line is parallel to the small diagonal.

To read more: Hermann-Mauguin symbols on Wikipedia (german only)