Our paper “Absolute energy level positions in tin and lead based halide perovskites” is published today on Nature Communications for publication. Here, we address one of the most debated topics in the material class of metal halide perovskites: the absolute energy levels of these perovskites and the physical origin of the changes in electronic band gaps of different perovskite compounds. We started about two years ago to address this problem, by initiating a large scale investigation of all 18 tin and lead based halide perovskites. We join forces in this study, where experimental research including materials preparation, characterization and photoemission spectroscopy were carried out in Germany led by Selina Olthof and theoretical analysis was done at our group in the Netherlands. Combining experiments and theory, we were successful in extracting a consistent and complete sets of energy level values. Further, by introducing a tight-binding model, we are also able to explain where the changes in energy levels come from. The findings are important for further material optimization, e.g. by mixing different perovskite components. The full paper is open access on Nature Communications; the peer review reports are open access and the stories behind the work are available at websites of Nature Communications and TU/e, respectively.