In his 1968 paper with James R. Holton, "A theory of the quasi-biennial oscillation", Lindzen presented his theory of the QBO after testing it in a two-dimensional (2-D) numerical model that had been developed by Holton and John M. Wallace. They showed that the QBO could be driven by vertically propagating gravity waves with phase speeds in both westward and eastward directions and that the oscillation arose through a mechanism involving a two-way feedback between the waves and the mean flow. It was a bold conjecture, given that there was very little observational evidence available to either confirm or confute the hypothesis. In particular, there was still no observational evidence of the westward-traveling "Kelvin" waves; Lindzen postulated their existence theoretically.