If the constituent particles of a unit cell are present at its body-centre besides the ones that are at its corners.
o Face-Centred Unit Cells:
If the constituent particles of a unit cell are present at the center of each face, besides the ones that are at its corners.
o End-Centred Unit Cells:
If the constituent particles of a unit cell are present the centre of any two opposite faces besides the ones present at its corners.