The energy positions of the visible and the infrared luminescence bands in porous Si are related. Their dependence on different parameters is examined. They are both blue-shifted (from 1.0 eV to 2.1 eV for the visible and from 0.8 eV to 1.3 eV for the infrared band) due to a reduced crystallite size. However, the visible band can be red-shifted by up to 300 meV by surface effects. The observed behaviour is characteristic of neither pure quantum systems nor amorphous alloys. It is typical for porous Si containing surface slates.