A monochromatic laser can be moved along a ruler. There are two diffraction gratings, one of which has a known number of lines. The diffraction pattern can be measured against a ruler (which is not at right-angles to the zero order, so the user must rotate it to correct the measurements). The user needs to accurately measure the positions of the orders to get the wavelength of the laser. Having calculated this the line separation of the second diffraction grating can be found.