Panel a: T_eff = 1600 K, log  g = 3.5 BT-SETTL model (green curve; Baraffe et al. 2015), smoothed to the resolution of our K-band spectrum HD 106906 b (black) and scaled to the same average flux. Most features observed in the spectra are reproduced by the (virtually noise-free) model, supporting evidence that the measured features are caused by atmospheric absorption rather than noise. Panel b: residuals. Panels c and d: same for a shorter wavelength range.