Atoms coordinates: Once an origin is fixed and given the basis vectors of the cell a, b and c, any atom is arranged in the structure by a vector T=xa+yb+zc, where x,y,z are the positional coordinates of the atom. These coordinates are fractions of the three reference basis vectors and their values have to be:  0 ≤ x, y, z < 1 (if the values are of type 0.25, 0.5, 0.75 they are written as 1/4, 1/2, 3/4 respectively).

Equivalent positions: Atomic positions whose coordinates are given by the action of the symmetry elements of the Space Group from a particular position.

Wyckoff positions: Positions in which atoms can be found in the structures. They include both the positions equivalent to a general position (position containing only the identity element) and the special reduced positions, generating a lower number of positions to the general position and which are located on the point symmetry elements of the Space Group.

P 1 21/c 1