text: Manne Siegbahn began his studies of X-ray spectroscopy in 1914. Initially he used the same type of spectrometer as Henry Moseley had done for finding the relationship between the wavelength of some elements and their place at the periodic system. Shortly thereafter he developed improved experimental apparatus which allowed him to make very accurate measurements of the X-ray wavelengths produced by atoms of different elements. Also, he found that several of the spectral lines that Moseley had discovered consisted of more components. By studying these components and improving the spectrometer, Siegbahn got an almost complete understanding of the electron shell. He developed a convention for naming the different spectral lines that are characteristic to elements in X-ray spectroscopy, the Siegbahn notation. Siegbahn's precision measurements drove many developments in quantum theory and atomic physics.