The unit cell is the absolute smallest repeating box of the solid crystal. When you stack this box repeatedly in three dimensions, it builds the entire massive crystal lattice. Scientists classify crystal systems by measuring the unit cell edge lengths (a, b, c).
They also measure the internal angles (alpha, beta, gamma) between those specific edges. These detailed measurements group all unit cells into exactly seven broad symmetry categories. The seven categories are cubic, tetragonal, orthorhombic, rhombohedral, hexagonal, monoclinic, and triclinic.
Chemists also look at where atoms actually sit inside the repeating structural box. Atoms might sit only at the outer corners, or they might sit perfectly in the center of the faces. Combining the seven basic systems with these different centering types creates the 14 Bravais lattices.
These 14 geometric patterns underlie all known crystalline materials in the entire universe. A material’s internal crystal structure directly controls many of its most important physical traits. The structure governs the solid density, how it shatters, and its unique optical properties.
It also determines the specific diffraction pattern the crystal creates when hit by X-rays. A common misconception is that amorphous solids like window glass have a real crystal structure. Standard glass lacks this repeating internal order, so it does not have a true unit cell.