Required resolving power (R  ≡  E/ΔE) for separating the Lyα_1, 2 (red), β_1, 2 (green), and γ_1, 2 (blue) fine-structure doublets of major elements compared to the instrumental resolving power of the HST STIS (Kimble et al. 1998), FUSE (Moos et al. 2000), Chandra LETG (Brinkman et al. 2000) and HETG (Canizares et al. 2005), XMM-Newton RGS (Den Herder et al. 2001), and XRISM Resolve (Ishisaki et al. 2022) using the FWHM. The elements below the curves can be resolved with the spectrometer. The symbol size represents the logarithm of the solar abundance of the elements (Anders & Grevesse 1989).