This paper explores, in some detail, the development of third person plural impersonal constructions into passive ones with the aim of determining the conditions most conducive to the emergence of a canonical passive, i.e. one which is both promotional (with an overt lexical subject) and agentive (with an overt agent). On the basis of cross-linguistic data it is argued that the required conditions are, on the one hand, the existence of a morphological alignment which does not distinguish between the O of a transitive clause and the S of a passive one and, on the other hand, the availability of highly grammaticalized third person plural impersonals, i.e. ones which can be used in episodic contexts and with different types of agents, among them individual and specific ones. The documented rarity of promotional passives originating from third person plural impersonal constructions is attributed to the rarity of the coincidence of the above two sets of independent factors.