A close packed unit cell, both face-centered cubic and hexagonal close packed, can form two different shaped holes.  Looking at the three green spheres in the hexagonal packing illustration at the top of the page, they form a triangle-shaped hole.  If an atom is arranged on top of this triangular hole it forms a tetrahedral interstitial hole. If the three atoms in the layer above are rotated and their triangular hole sits on top of this one, it forms an octahedral interstitial hole. In a close-packed structure there are 4 atoms per unit cell and it will have 4 octahedral voids (1:1 ratio) and 8 tetrahedral voids (1:2 ratio) per unit cell. The tetrahedral void is smaller in size and could fit an atom with a radius 0.225 times the size of the atoms making up the lattice.  An octahedral void could fit an atom with a radius 0.414 times the size of the atoms making up the lattice. An atom that fills this empty space could be larger than this ideal radius ratio, which would lead to a distorted lattice due to pushing out the surrounding atoms, but it cannot be smaller than this ratio.