A theory of the general circulation which admits mid-ocean fronts is examined. This study is related to some earlier studies of two layer planetary geostrophic systems, where it was argued that interior discontinuities, or “fronts”, generally arise as a result of the nonlinearity in the continuity equation. Similar dynamics are studied here under more general circumstances. It is argued that the most general solution for the interior thermocline requires several fronts, each of which arises in response to a mismatch of subsurface potential vorticity structure on isopycnals. Further, these fronts can influence each other and determine large areas of the basin scale potential vorticity field. Comparisons between special analytical solutions and numerical solutions are made. The main result of this work is that the frontogenisis in planetary geostrophic dynamics is robust, characteristic of highly stratified systems and of potential importance to basin scale structure.