Rest-frame optical emission lines in the G395M medium-resolution spectrum of GS-z9-0. The best fit to continuum and emission lines is overlaid in red; detected lines are highlighted (in green marginal ∼3σ detections). The left-hand panel shows a zoomed-in image of the region between [O II] λλ3726,3729 and Hδ, highlighting the detection of both strong lines and fainter Balmer lines such as H9 and H10. The middle panel shows the region around Hγ and the [O III] λ4363 auroral line, while the right-hand panel highlights the high S/N detections of the [O III] λ4959,5007 and Hβ complex.