Example results of the algorithm used to match GMCs (blue) and H II regions (red) for the same field of view. The background image is Hα line emission from a spiral arm segment in NGC 4254. The right and left panels show the matches obtained when the area of the GMC–H II region overlap is at least 10% and 70%, respectively, of the projected area of a GMC. Imposing a higher overlap percentage leads to fewer identifications of matched GMC–H II regions.