The correspondence started with a discussion of defects in chalcogenide glasses. Mott contacted Street because his work on luminescence provided evidence for a negative correlation energy property that Phil Anderson had proposed. Mott was interested whether this property applied to defect states in chalcogenides. The letters start when Street moved to the Max Planck Institute in Stuttgart in September 1974. He did not keep any previous letters. Street and Mott developed an explanation for defects that resulted in the paper (Phys. Rev. Lett.,35, 1293, 1975) and Mott, Davis and Street (Phil. Mag. 32, 961 1975) referred to as MDS. The topic of the letters broadened to other aspects of the electronic properties of amorphous semiconductors. In 1976, Kastner Adler and Fritsche (Phys Rev. Lett. 37, 1504, 1976), referred to as KAF, published a similar but competing model for defects in chalcogenide glasses. Several letters, including to Marc Kastner are about this controversy.