Relation between the break energy E_break and the index α₁ of the power law describing the spectrum below E_break. The sample includes all time-resolved spectra of the ten long GRBs analysed in this work. The dashed black horizontal line indicates a difference in behaviour, with spectra below this line having very hard and ill-constrained best fit values of α₁. All the parameter distributions and their mean values and standard deviations presented in this work rely only on spectra with E_break larger than this threshold value (blue points). The majority (∼85%) of the spectra with E_break <  20 keV (red points) belong to one specific GRB, namely GRB 171010 (orange points). The mean values of E_break and α₁ (along with their average errors) are represented for each sample with the solid black lines. The low-energy threshold of the GBM NaI detectors is shown with a grey solid line.