The crystal is built up of identical units, called unit cells, which are stacked together so that, in the absence of imperfections, they completely fill the space occupied by the crystal. The unit cell is a rhomboid (each face is a parallelogram, and opposite pairs of faces are equal) with side lengths a, b and c. The angles between the sides are denoted by the Greek letters α, β and γ, where α is the angle between sides b and c, β between c and a, and γ between a and b.[1] For vauxite, the reported values of these parameters differ slightly from reference to reference, as different researchers have studied different samples, but all agree that a = 9.1 Å, b = 11.6 Å, c = 6 Å, α = 98.3°, β = 92° and γ = 108°, to the accuracy stated.