They are classified as novices, journeymen, dilettantes or masters. They are Britain's hit men — killers who ply their deadly trade in return for cash, and who for the first time have become the subject of a major academic study.
The killers typically murder their targets on a street close to the victim's home, although a significant proportion get cold feet or bungle the job, according to criminologists who examined 27 cases of contract killing between 1974 and 2013 committed by 36 men (including accomplices) and one woman.
The publication of the research is chillingly timely: on Friday, high-flying financial executive Robin Clark was shot in the leg in a targeted hit by a masked gunman as he got out of his car at a railway station in Essex, southeast England.