Crystalline solids have long-range, three-dimensional order. The smallest repeating unit is the unit cell. Seven crystal systems based on edge lengths (a, b, c) and angles (alpha, beta, gamma): Cubic (a=b=c, all 90), Tetragonal (a=b!=c, all 90), Orthorhombic (a!=b!=c, all 90), Hexagonal (a=b!=c, alpha=beta=90, gamma=120), Monoclinic (a!=b!=c, alpha=gamma=90, beta!=90), Triclinic (a!=b!=c, no 90), Rhombohedral (a=b=c, alpha=beta=gamma!=90). Total 14 Bravais lattices. For JEE: focus on the cubic system — SC, BCC, FCC.