The article discusses the historical context and correct attribution of the Scherrer equation, which is used to determine the mean size of single-crystal nanoparticles or crystallites in nanocrystalline materials. The equation was developed by Paul Scherrer in 1918, while working with his supervisor, Peter Debye, at the University of Göttingen. The authors clarify that the Scherrer equation is not the 'Debye–Scherrer equation' and provide a detailed explanation of its derivation and limitations. They emphasize that the equation is applicable for average sizes up to about 100-200 nm due to the decreasing effect of peak broadening with increasing crystallite size. The article also highlights the contributions of Albert Hull and the importance of recognizing the various factors that contribute to X-ray diffraction peak broadening.The article discusses the historical context and correct attribution of the Scherrer equation, which is used to determine the mean size of single-crystal nanoparticles or crystallites in nanocrystalline materials. The equation was developed by Paul Scherrer in 1918, while working with his supervisor, Peter Debye, at the University of Göttingen. The authors clarify that the Scherrer equation is not the 'Debye–Scherrer equation' and provide a detailed explanation of its derivation and limitations. They emphasize that the equation is applicable for average sizes up to about 100-200 nm due to the decreasing effect of peak broadening with increasing crystallite size. The article also highlights the contributions of Albert Hull and the importance of recognizing the various factors that contribute to X-ray diffraction peak broadening.