Field emission from two special kinds of emitters has been studied in detail: intentionally introduced particles of controlled geometry, and sites produced by intentional mechanical damage to the cathode surface. We found that the size of particles seems to play no role in their threshold field but their shapes are a determinant factor since spherical particles do not emit for fields up to 120 MV m-1. The method of creation of damage sites, and the similarity of their emission on Nb and Au substrates, suggest the possibility that emission comes from geometrical protrusions. A model of superposed geometrical protrusions is proposed to explain the enhanced field emission behaviour of this particular type of surface defect.