According to an example, a first layer of a substrate comprises a plurality of gratings having a periodicity P. A second layer of the substrate comprises a plurality of gratings, overlapping with the first set of gratings, and having a periodicity of NP, where N is an integer greater than 2. A first set of gratings has a bias of +d and the second set of gratings has a bias of −d. A beam of radiation is projected onto the gratings and the angle resolved spectrum of the reflected radiation detected. The overlay error is then calculated using the angle resolved spectrum of the reflected radiation.