which describes the reconstruction of the (hkl) plane (given by its Miller indices). In this notation, the surface unit cell is given as multiples of the nonreconstructed surface unit cell with the unit cell vectors a and b. For example, a calcite(104) (2×1) reconstruction means that the unit cell is twice as long in direction a and has the same length in direction b. If the unit cell is rotated with respect to the unit cell of the nonreconstructed surface, the angle φ is given in addition (usually in degrees). This notation is often used to describe reconstructions concisely, but does not directly indicate changes in the layer symmetry (for example, square to hexagonal).