Broad emission lines in active galaxies: horns, bumps and shoulders

It has become increasingly clear that the broad emission lines of active galaxies often have complex and highly structured profiles. We discuss, in the context of the discrete cloud model of the broad-line region, three possible mechanisms by which profile structure could be produced: irregularities in the radial luminosity distribution, flattened geometries and ionizing continuum variability. In each case, the structure produced has distinctive qualities which can be compared with the variety of features present in observed broad emission-line profiles. (author)