We have determined the stability, optimun geometry, and electronic properties of nanometer-sized capped graphitic cones, called “nanohorns”. Different nanohorn morphologies were considered, which differe in the relative location of the five terminating pentagons. Simulated scanning tunneling microscopy images of the various structures at different bias voltages reflect a net electron transfer towards the epntagon vertex sites. We found that the density of states at the tip, ovservable by scanning tunneling spectroscopy, can be used to discriminate between different tip structures.