An oblique cell is compatible with the presence of axes 1 or 2 normal to the cell. This cell is primitive and has point group 2.

If a reflection line m is in the plane, two cases can occur:

a) the cell is rectangular, primitive

b) the cell is oblique and primitive, with a = b and gamma not equal to 90. In this case a rectangular (more convenient) centred cell may be chosen defined by the vectors a' and b'.

The presence of a fourfold axis normal to the plane restricts the possible unit cell to the square.

The presence of a three- or sixfold axis normal to the plane restricts the possible unit cell to the hexagonal type.