Ludwig Wittgenstein and Rudolf Koder met as elementary school teachers in Puchberg (Lower Austria). Their lifelong friendship primarily grew from a shared interest in music, which serves as a recurring theme in their correspondence published here for the first time, spanning from 1923 until Wittgenstein's death in 1951. The editor used the correspondence as an opportunity to explore the musical influences in the Wittgenstein family and in the life and philosophy of Ludwig Wittgenstein in two essays.