A model of cortical folding in mammals is presented. The model consists of a cube, superimposed on which are straight close-packed gyri, running the length of the cube. The cortex is represented by a thin layer of constant thickness. It proves possible, by adjusting the length, height, and width of the 'gyri' and the thickness of the 'cortex', to obtain a reasonable fit to the available empirical data (which extend over three to four orders of magnitude in brain weight). The model directs attention to possible features of the macroscopic organization of the mammalian brain which are novel and hitherto unremarked.