The quest to understand the way in which neurons interconnect to form circuits that function as a unit began when Ramon y Cajal concluded that axo-dendritic apposition were too conspicuous to be incidental and proposed that two neurons must be communicating through these points of contact (see Shepherd and Erulkar, 1997, Trends Neurosci., 20, 385-392). Lorente de Nó was probably the first to predict that a defined group of vertically displaced neurons in the neocortex could form functional units (Lorente de Nó, 1938, Physiology of the Nervous System, 20, OUP: 291-330) for which Mountcastle found experimental evidence (see Mountcastle, 1997, Brain, 120, 701-722) and which was ultimately demonstrated by Hubel and Wiesel in their elegant discovery of the orientation selective columns (Hubel and Wiesel, 1959, J. Physiol., 148, 574-591). Until today, however, it is still not clear what shapes functional columns. Anatomical units, as in the barrel cortex, would make it easier to explain, but the neocortex is largely a continuous slab of closely packed neurons from which multiple modules emerge that can overlap partially or even completely on the same anatomical space. Are the columns in fixed anatomical locations or are they dynamically assigned and what anatomical and physiological properties are operating to shape their dimensions? A recent study explores how the geometry of single neurons places structural constraints on the dimensions of columns in the visual cortex (Stepanyants et al., 2008, Cereb Cortex, 18, 13-24).