Normalized optical and NIR spectra (from the z to the K band) of five representative metal-poor ultracool dwarfs: the sdT2 WISE0004 from Group I in the colour–colour diagram shown in Fig. 1 (the optical part from this research and the NIR part from Greco et al. 2019); the Z-class prototype WISE1810 from Group III (the optical part from Lodieu et al. 2022 and the NIR part from Schneider et al. 2020); the sdT4 WISE1553 from Group II in between Group I and Group III (the only NIR spectrum from Meisner et al. 2021); the sdL8 2MASS0645 (Zhang et al. 2018); and the esdL7 2MASS0532 (Burgasser et al. 2003b). The major absorption bands and atomic lines are marked.