A target structure, wherein the target structure is configured to be measured with a metrology tool that has a diffraction threshold; the target structure including: one or more patterns supported on a substrate, the one or more patterns being periodic with a first period in a first direction and periodic with a second period in a second direction, wherein the first direction and second direction are different and parallel to the substrate, and the first period is equal to or greater than the diffraction threshold and the second period is less than the diffraction threshold.