A unit cell of  ccp  or  fcc  lattice is divided into eight small cubes. Then, each small cube has 4 atoms at alternate corners. When these are joined to each other, a regular tetrahedron is formed.

Cnt undrstnd it.. That how tetrahedron is formed on joining them???

tetrahedron is formed by joining four atoms,,.. 

there are 4 atoms  in each small cube,

because in ccp structure there are 4 atoms , they have thus taken atoms alternately .

one tetrahedron makes one tetrahedral void in each small cube , and there are 8 such cubes in the unit cell ,therefore total tetrahedral voids becomes 8.

and as it is CCP lattice there are 4 atoms as whole  therefore no. of tetrahedral voids is twice the no. of atoms ..