Tetrahedral voids can be found in substances having a tetrahedral arrangement in their crystal system.
In a cubic, close-packed structure, spheres of the second layer lie above the triangular voids of the first layer.
Each sphere touches the three spheres of the first layer.
By joining the centre of these four spheres, it forms a tetrahedron, and the space left over by joining the centre of these spheres forms a tetrahedral void.
Hence, the number of spheres that form a tetrahedral void is four, 3 in one plane and 1 in the axis.