(N/A) unit cell is characterized by six parameters:
$1$. Its dimensions along three edges $a, b$,and $c$. These edges may or may not be mutually perpendicular.
$2$. Angles between the edges: $\alpha$ (between $b$ and $c$),$\beta$ (between $a$ and $c$),and $\gamma$ (between $a$ and $b$).
Unit cells are broadly classified into two categories:
$(a)$ Primitive Unit Cells: When constituent particles are present only on the corner positions of a unit cell,it is called a primitive unit cell.
$(b)$ Centred Unit Cells: When a unit cell contains one or more constituent particles present at positions other than corners in addition to those at corners,it is called a centred unit cell. These are of three types:
$(i)$ Body-Centred Unit Cells: Such a unit cell contains one constituent particle at its body-centre besides the ones at its corners.
$(ii)$ Face-Centred Unit Cells: Such a unit cell contains one constituent particle present at the centre of each face,besides the ones at its corners.
$(iii)$ End-Centred Unit Cells: Such a unit cell contains one constituent particle at the centre of each of two opposite faces,besides the ones at its corners.