Theoretically, to determine the total abundance of a single element in an H region, all transition lines should be observed and summed. However, this can be observationally difficult due to variation in line strength. Some of the most common forbidden lines used to determine metal abundances in H regions are from oxygen (e.g. [O ] λ = (3727, 7318, 7324) Å, and [O ] λ = (4363, 4959, 5007) Å), nitrogen (e.g. [N ] λ = (5755, 6548, 6584) Å), and sulfur (e.g. [S ] λ = (6717, 6731) Å and [S ] λ = (6312, 9069, 9531) Å) in the optical spectrum, and the [O ] λ = (52, 88) μm and [N ] λ = 57 μm lines in the infrared spectrum. Oxygen has some of the stronger, more abundant lines in H regions, making it a main target for metallicity estimates within these objects. To calculate metal abundances in H regions using oxygen flux measurements, astronomers often use the R 23 method, in which