Thalamocortical (TC) afferents give rise to the elementary functional units of sensory cortex, cortical columns. Principles underlying spread of inhibition within a column remain, however, unknown. Here, we unravel how TC input may be relayed spatially by inhibitory interneurons (INs) by classifying axonal projection patterns of INs (N=204) located throughout layers (L) 2-6 in rat vibrissal cortex (vS1). We found five projection types independent of laminar location and postsynaptic target specificity. We integrated these into a dense average 3D model of rat vS1, thus allowing describing TC input to INs by structural overlap. This procedure provided first-order quantitative estimates of how TC input is relayed by INs within a column. We found three major TC->IN pathways that are largely decoupled from those of excitatory cell types: (i) focal inhibition to L4, (ii) blanket inhibition, and (iii) specific inhibition targeting cortical zones of highest IN soma density
