Langacker (1987, 2008) defines metonymies as conceptual shifts within a domain or domain matrix. However, there are several cases in which metonymical shifts between conceptual entities that belong to the same domain are not possible. Thus, in this paper a more restrictive definition of metonymy is developed on the basis of frames, understood as recursive attribute-value structures. It is claimed that metonymies can be explained by a simple frame transformation requiring a necessary condition that I refer to as bidirectional functionality. This assumption is confirmed by an analysis of metonymical processes in various common types of word formation in German, including possessive compounds, -er nominalizations, and synthetic compounds. Furthermore, bidirectional functionality seems to underlie a sub-class of nominal compounds I suggest calling "frame compounds".