Fractional coordinates
![Unit cell definition using parallelepiped with lengths a, b, c and angles between the sides given by α,β,γ[1]](/Images/godic/202501/14/UnitCell4457.png")
In crystallography, a fractional coordinate system is a coordinate system in which the edges of the unit cell are used as the basic vectors to describe the positions of atomic nuclei. The unit cell is a parallelepiped defined by the lengths of its edges a, b, c and angles between them α, β, γ as shown in the figure below.