A spectrum of Sn in the region around 135 A arises from transitions in Sn ions with the 4d electrons in the ground configuration. Because of the very complex structure of these transitions, the spectrum is largely unknown. Only four lines were identified in Sn XIV, where the ground configuration is 4p64d. We made calculations of the energy levels and transition probabilities in the Sn VI–XV spectra using the Flexible Atomic Code (FAC). Some results of similar calculations made with the aid of the Cowan code also can be found in Chapter 5. The spectrum of Sn excited in a low-inductance vacuum spark was recorded on a high-resolution grazing-incidence spectrograph in the region 100–200 A and compared with the results of the calculations. Due to the large uncertainty of the calculations, the straightforward identification of the experimental spectrum is impossible; but some general features can be traced.