As you can see now, the choice of unit cell is not unique and we can define any unit cell of any shape as long it contains one lattice point.
However, as one starts defining various shapes, we come up with seven categories, called as crystal systems, in which all possible unit cells shapes would fit provided space filling criteria is fulfilled.
Seven crystal systems are shown below.
Crystal system and lattice parameters
Minimum symmetry elements
Cubic
a = b = c,
Four 3-fold rotation axes
Tetragonal
One 4-fold rotation (or rotation-inversion) axis
Orthorhombic
Three perpendicular 2-fold rotation (or rotation-inversion) axis
