The reciprocal lattice of a 3D real lattice is constructed
in the same way as for 2D lattices, with the addition of a third unit vector, c*, which is defined:
c* points in
a direction
perpendicular to a and b.
An orthorhombic unit cell is shown opposite
in both its real and reciprocal space representations. Change the size of the real cell by
dragging the white lattice points, and observe the effect on the
reciprocal lattice cell. 

In
the lattice shown above, the reciprocal vectors are parallel to their corresponding real
vectors. This is only true for the unit cells of certain crystal systems, which are they?