Hi all, I read The unit cell is the smallest structure that repeats itself by translation through the crystal. Some says premitive unit cells contains atoms only at the corners while a unit cell may contain extra atoms in between(like bcc or fcc). At one place I found this: For each crystal structure there is a conventional unit cell, which is the smallest unit that has the full symmetry of the crystal (see below). However, the conventional unit cell is not always the smallest possible choice. A primitive unit cell of a particular crystal structure is the smallest possible unit cell one can construct such that, when tiled, it completely fills space. This primitive unit cell does not, however, display all the symmetries inherent in the crystal. A Wigner-Seitz cell is a particular kind of primitive cell which has the same symmetry as the lattice. I am confused what does they mean by premitive unit cell and unit cell. If the only difference between them is of the extra atoms. Then for a given crystal the volume of a premitive unit cell and a unit cell must be same. Or there be anything smaller than unit cell, but above what if found says "However, the convention unit cell is not always the smallest possible choice."Please explain. Also at many place people say a premitive unit cell contains only one lattice point. I am unable to comprehend this because of a fact for example I know in simple cubic 8 lattice points(not one) occupied by 8 atoms. Any help will be highly appreciated.