On the left is a rectangular space lattice and on the right (activated or hidden by clicking on the lattice button above the frame) is the corresponding reciprocal lattice. Move the sliders defining the space lattice to see whether the reciprocal lattice changes as you expect. Don't forget to vary phi as well as a1 and a2.
Click on the show planes button and then DOUBLE-click near one of the reciprocal lattice points. The origin of reciprocal space and the selected reciprocal lattice point are highlighted and the coordinates (h k) of the selected reciprocal-lattice point appear underneath. In the space lattice, two adjacent planes of the (h k) set are drawn. Recognition of the correspondence between points in reciprocal space and planes in the crystal space is essential to understanding the Laue picture of the x-ray diffraction experiment.