There is an increasing awareness of the high frequency of field placement errors occurring in radiotherapy. If such errors are to be rectified systematically to provide a sustainable improvement in field placement accuracy over a course of treatment, the origins of the errors require unambiguous identification. From an examination of the procedures taking place between the exposure of the prescription and treatment films, we propose resolving field placement errors into four groups: patient motion, entrance field location, field shape or size, and beam direction. We also suggest means by which the components of clinically observed field placement errors may be resolved in practice.