 Matrix mechanics and wave mechanics were formulated roughly around the same time between 1925 and 1926. In July 1925, Heisenberg finished his seminal paper “On a Quantum Theoretical Interpretation of Kinematical and Mechanical Relations”. Two months later, Born and Jordan finished their paper, “On Quantum Mechanics”, which is the first rigorous formulation of matrix mechanics. Two months after this, Born, Heisenberg, and Jordan finished “On Quantum Mechanics II”, which is an elaboration of the earlier Born and Jordan paper; it was published in early 1926. These three papers are reprinted in van der Waerden (1967). Meanwhile, Schrödinger was working on what eventually became his four famous papers on wave mechanics. The first was received by Annalen der Physik in January 1926, the second was received in February, and then the third in May and the fourth in June. All four are reprinted in Schrödinger (1928).